Nah, you're not missing anything, and that is already happening. There's a few users who keep creating new accounts on alternate instances to post troll threads. But it's on a small-enough scale that it's trivial for mods/admins to deal with. And realistically, it's more work for the troll than it is for the mods, so eventually the troll user will either tire themselves out or simply run out of instances to make new accounts from.

It's definitely an intrinsic problem that ActivityPub doesn't really have a reasonable solution for yet. It's sort of a necessary evil in order for the rest of the Fediverse to function freely, and part of the compromise that users and communities need to accept and live with in order to keep things running. But, I don't think it'll become a major issue unless the Fediverse sees a massive influx of users, like the sort of scale that Bluesky saw with Twitter's exodus.