But apparently what they actually meant was, “users of Mastodon instances rarely explore outward”? The instances would auto-federate, but in practice, the “crawlers” (the users) aren’t leaving their bubbles often enough to create a critical mass of interconnectedness across the Fediverse?

It's more along the lines of, as Mastodon's been one of the more popular ActivityPub platforms for awhile longer, there's a longer history of federation faffery, i.e. instance admins/people not getting along leading to defederations leading to a somewhat more fragmented network. Lemmy's only grown in adoption more recently and hasn't had as much time for that faffery to crop up as much, and has a different style and audience to it anyway, so it may be less prone to that, time will tell.

Regardless, your conclusion is basically on point for many folks. Federation stuff is no better to them than the erratic moderation/management of larger platforms that's driving them elsewhere. Of course problem is, moderation/management's not really something tech can solve (as Bsky's already run into with its attempts at enabling third-party moderation).