a) a lot of accounts are bots, and depending on how they are implemented, a LOT of these have remained (or even were created) after the API changes - remember, it's easy to spin up 1000s of these to each provide small traffic so as to not run up against the API limits. Overall, I suspect a ton more bots are there now, b/c the bot defense effort was suspended, b/c unlike a single bot, that one needs to look at ALL traffic (I suppose it could be re-written from scratch in a decentralized manner but... the developers did not choose to do that).

b) a lot of people who remain on Reddit, including myself, offer it WAY less traffic than before. I used to be a mod of a small sub, which I quit, so I went from checking it almost literally hourly, so at most once a day, and most days I do not even comment at all. Also, I used to browse r/all (actually, "popular"), but now I never do, instead preferring Lemmy/Kbin for that. My personal traffic dropped off a cliff just like this image shows, in fact probably a lot more so. Although I still do visit that small gaming sub, b/c while there is a version of it here, instead of like 5 posts a day we get at most 1 per week, which less than a handful interact with. So that is not an "exodus of users" so much as a (vast) reduction of interaction, which still impacts their advertising revenue and thus the continuity of Reddit as a corporate entity.

c) as people are saying, not everyone came to Lemmy/Kbin. Some went to Mastodon, others just stopped going online as much, and like myself I comment now a lot less than I used to, though I read just as much (here, not there). So just b/c the traffic did not come "here", does not mean that it did not leave "there". i.e., think of the shock of the event as making people regress more to lurking and not feel as comfortable interacting, especially given the lack of ability of smaller magazines (what are those called on Lemmy again?) here. Thus, even if they did not "go outside", they still may not be interacting on Reddit.