I’ve heard it from native peoples, on a couple of the reservations near here. But they certainly didn’t mean it to be offensive.

Lack of offensive intent doesn't seem to matter though if someone feels "offended" by a term of differentiation.

Sometimes, even with the best of intentions, people just get it wrong. Or society advances so that what was ok is now no longer ok. Really all that's required is that we are sensitive to these changes and if we are asked to change ourselves in ways that can only do good, or at worst can do no harm, then why would we not do it? There are so many cultural sensitivities around the world that we will probably always get something wrong for somebody, but that doesn't mean that we should stop trying to do better overall. (I'm not suggesting that you are suggesting that, btw, just making a general point.)

Ethnicity, in particular, is a minefield because of the way it has been, and still is, used to cause harm.

I have been familiar with the use of Anglo to mean English speakers, used mostly by Latinx writers, for some time. I haven't really paid much attention to any of its connotations, until I was in the process of planning to use a writing with the word in a class. So I was doing close reading, paying attention to meaning, and connotation, more that I had in the past.
The first few readings it grated a little as I couldn't rid myself of the ethnic use of the word, it took a few read throughs before I could get past my cultural understanding of the word and get to the writers intended meaning (I knew their intention, but I still had my cultural baggage). I have never seen any intention to cause insult from its use.
I was wondering if there was any, or could be any offence, by its use, unintended, to some groups. I was particularly thinking of the reaction of Irish people, if they were called Anglo. As I would want to warn, that under some scenarios it had the potential to offend.
It doesn't look like anyone here sees any possibilities of offence from its use. Which is good and dandy. Though I might just add that the possibility exists, as we haven't had any Irish (although we have had British-Irish input) input.
Edit: I am still not 100% sure if it means all speakers of English, or white speakers of English. Most seem to go with whites speakers of English, but not everyone seems to be in agreement about that.

Oh I'm sure it contains the potential to give offence, in particular because of it admittedly unclear ethnic connotations, and also political ones.

True. A few years ago someone at work was offended when the word "black" was used (even though its use was common and the person certainly meant no offence). The right thing to do in such cases is simply to take care to use the word expected by the other party (in this case "African-American").

Whereas in the entirely different social and historical context of Mexico, "black" or "white/pale" are used as affectionate terms.

Never quite sure what the preferred term is these days. Here's my understanding, feel free to correct me if I am mistaken.

USA
was 'black'
then only 'African American'
now 'black' preferred 'African American' ok
UK
black .

For a short period here in the US, 'black' was considered to be a bad term, and 'African American' was widely used. Then 'black' came back into common usage. I think there was a realization that there were many blacks who were not 'African Americans' (visitors from Africa who were not at all 'American', people from Jamaica, the Caribbean, etc). I don't hear the term 'African American' at all these days.

That's helpful, thanks. I suppose after BLM, 'black' is now a uncontroversial term. I remember when 'African American' first came into vogue and thinking it was quite a mouthful.

I agree with most of that, except I don't think African American has been phased out in my neck of the woods, but trying to decide whether the "appropriate" adjective is "black" or "African American" seems to be a minefield as it seems to be random as to which one might set someone off, though in fairness, among the people I meet at work or otherwise [1] it is rarely a relevant question, and [2] they are all good natured enough to accept that most people don't have bad intent. .... IOW either term, and of course others, becomes offensive when the context in which it is used is as a euphemism for "those people!"

Strictly answering your question. Used by Latinos in this context, it just means white people. Specifically, white North Americans. It is derived from the original Spanish-language invaders and they would have been referring to Anglia and/or the English. It is not meant, or received, as a pejorative. My feeling is that today it is actually meant in slightly gentler way than just saying "white guy/lady" but I'm not backing that up with any science.

The history of the Spanish Conquistadors was all happening along side (and often overlapping) with the English colonization of what would become America/USA. It was probably more succinct originally and shifted into almost slang.

I tried doing some Spanish-language googling on the subject, but it kept giving me results for "angeles" instead. Not at all the same thing.

I don't know that I've really heard the word used as a collective noun in Mexico, or Mexican communities. What I have heard of course is "gringos/as", a word of uncertain etymology but in my experience always as a "light" pejorative in the sense that I've never heard a compliment delivered regarding "gringos" but many generalisations and as a constant casual reference. I've had many an interesting debate with my Mexican friends around their use of this term - it's so ubiquitous there that they barely notice that they are applying a pejorative stereotype until some annoying person like me asks them about it. I do get the sense that it largely means "white citizens of the US", and they do pause when asked whether or not it includes other people of other ethnicities who are born and bred in the US.

I would think it's more used among long established Latino communities within the US, rather than in Mexico. Even then, not excessively. The word probably developed out of a need for indigenous people (that were there long before the Spanish and/or the English) to differentiate between two different groups of white people. The need wouldn't have arose within Mexico --- it was all Spanish. In the US it ultimately became all English in terms of who was in charge.

It's interesting to think about. Travelling around the US, observing street, city and state names, it's obvious which areas were Spanish speaking.