Would you say smaller forums where people largely know each other are communities then? IRC? Discord?

Probably not, but they're at least closer. Real communities provide you care, support, relief from loneliness, a sense of purpose, etc. etc. etc.

It's possible for some (lucky souls) to find tiny nuggets of these benefits in even the worst online "communities" (I think partially because we're hard wired as humans to need these things), but by and large it's does not exactly scratch the same itches that your grandma's sewing circle or bridge club used to.

Because I struggle to think what else could or has ever fit such a strict definition.

It's difficult to reason about because if you're anywhere close to my age group (old ass millenial) online "communities" appeared and replaced existing physical communities across the country (I'm speaking in US terms). We're now basically as lonely as we've ever been as a country, and I think it's at least partially related to us going inside and screen timing it up for a number of decades on these platforms where "the community" is a bunch of strangers angrily typing messages to you through the Internet.

I find it no small coincidence that loneliness in America skyrocketed even as people became more active on social media. It points at the exact lack of benefit you get out of these "communities" that you used to get out of the old type.