Part of what makes Twitter, Reddit, etc. such easy targets for bot spammers is that they're single-point-of-entry. You join, you have access to everyone, and then you exhaust an account before spinning up 10 more.
The Fediverse has some advantages and disadvantages here. One significant advantage is that -- particularly if, when the dust finally settles, it's a big network of a large number of small sites -- it's relatively easy to cut off nodes that aren't keeping the bots out. One disadvantage, though, is that it can create a ton of parallel work if spam botters target a large number of sites to sign up on.
A big advantage, though, is that most Fediverse sites are manually moderated and administered. By and large, sites aren't looking to offload this responsibility to automated systems, so what needs to get beaten is not some algorithmic puzzle, but human intuition. Though, the downside to this is that mods and admins can become burned out dealing with an unending stream of scammers.