A "51% attack" isn't really meaningful for something like the Fediverse because there's no concept of any particular instance or group of instances being "authoritative." There's no special benefit to be had from owning a majority of the instances or users or whatever other metric you want to measure by.

If tomorrow Reddit were to magically federate, it would instantly have the majority of threaded conversation going on in the Fediverse under its control. If the day afterward it defederated again, it wouldn't mean that it had somehow "become" the Fediverse and the rest of us were being shed like irrelevant detritus. It's nothing at all like a cryptocurrency fork, where there's a strong incentive to follow whatever the "majority" fork is doing because that's where the money is.