very surprised to not see the word 'reddit' mentioned once in the piece (I already count 20 mentions here in this thread).

It's huge, it's structurally probably the closest to the traditional forum experience and my suspicion is it's eaten up a lot of former communities. One inherent appeal to a platform like that is that you have one identity across all instances which is appealing for practical reasons (nobody wants to manage a dozen accounts any more), and there's interaction across forums.

That last part is pretty important I think. Forums are great for the thing they deal with but they're also very silo-ish. With people nowadays preferring to exist in a lot of different communities that's often a disadvantage.