True, though the Mastodon compatibility is a bit iffy at the moment; they use ActivityPub in sufficiently different ways that it's not really possible to meaningfully access a Lemmy community via a Mastodon client, for example. I believe if it works at all, you'll basically just see a firehose of comments/posts.

And at those times, compare to now, there were hurdles to finding communities (discoverability) and having them interlinked in a “whole is greater than sum of parts” fashion.

I think Mastadon actually addresses a little bit of that, but also the discoverability especially is going to be difficult with the major search engines having vested interests that don’t align with surfacing results from less centralized platforms.

For example I have never once had a search return anything in the first few pages that links to a Mastadon instance, and can’t even remember that last time I had a useful result from of conversations from a traditional style forum, but I know they still exist because I can find them if I explicitly search for that sort of thing