OK, so, after looking it up, it seems that like Tofugu said in that article, it is possible to use で that way, but it only appears in one of 3-4 monolingual dictionaries I searched (namely 明鏡国語辞典, 3rd edition). The essence of it is that this sort of で specifies limits on the context in which something is occurring or being discussed. However, here’s the thing: most of the dictionaries discuss this usage for time expressions only.
Now, I think we could see this as an extension of the locative usage of で, which allows us to specify where something is happening and therefore to indicate a context, but I’m far more used to seeing this sort of ‘figurative location’ usage with things that can actually be construed as such (i.e. as areas, fields, containers/repositories etc). That’s why using it for スポーツ doesn’t strike me as much – it’s just a field. Additionally, sometimes, these things feel like an extension of the ‘means’ usage, which is how I understand it when I see stuff like the structure AとBとでは (‘with both A & B’ i.e. if that’s what I’m considering), though that doesn’t always work and can make me question whether or not we’re dealing with a て-form. It’s been so long since I’ve seen で（て-form of だ）は though, so I’m not very sure. But just to round up this thought, I very rarely see this being used for things that do not seem to encapsulate some sort of space in and of themselves. That’s why it strikes me as strange to use it with 〜山 or 〜人, even though it seems, based on what I’ve just read, that this usage is possible. (I will note, however, that even the dictionary that gave me this definition uses this as an example with 人: 「若い人の間で人気がある」)
Even so, I still feel – especially when the category evoked is not explicitly something that contains or encompasses something else – that it’s possible to parse this as the て-form of だ. I was hesitant to do so because I haven’t been consciously looking out for で as a て-form in quite a while, but it seems that one can see it this way. For example, in では, according to the dictionary on Goo:
【一】《断定の助動詞「だ」の連用形＋係助詞「は」》判断の前提を表す。…であるとすれば。…だと。「雨—中止になる」「彼—だれも承知しないだろう」
(In essence, it says that では designates something as a pre-condition for a certain judgement, rather like ‘if that’s the case’.) I think the で in でも is also very much closer to the て-form of だ than the particle in some cases.
Anyhow, I guess we can use both interpretations and get ‘among ~’ as a possible meaning. I agree that we could see that comma as a 読点, but my impression of commas in Japanese is that they’re very much up to the writer’s discretion, meaning that here, it could have been added in order to separate that first bit from the rest of the sentence, or in order to separate it from the next clause in order to avoid a long string of unpunctuated words. I haven’t got very far in my book on Japanese writing just yet, but I’m pretty sure both purposes are fine.
To round everything off, I’ll just say this: I guess either interpretation is valid, and perhaps mine ends up being the clunkier of the two – though I will say that extremely long relatively clauses are completely normal in formal written Japanese, so perhaps I’ve just got used to them – but if we really wanted to ensure that the ‘among ~’ or ‘with regard to ~’ nuance is expressed, what should be used – unlike in English, where a comma suffices – is a は:
英語が母国語でない人では 、ネイティブレベルや上級の英語を話す人もいれば、そうでない人もいます。
This is unequivocally a sentence that puts ‘people [for whom] English is not [their] native language’ (literally) front and centre, and makes a remark on that subject. That’s what は is for when it’s alone: topic indication, and at times, emphasis.
