In this case, generating fake excerpts is not something a user on a server controlled by someone else can do; they have to operate a malicious server themselves. Defederation is a good solution to malicious servers.

Certainly someone very determined could spin up a bunch of malicious servers and put out a bunch of posts containing fake excerpts, but they'd need followers to get any reach on the microblog side of the fediverse. They could spam Lemmy communities, but users would notice and downvote/report the posts.

So I think "just defederate" probably is an adequate solution here, at least as things currently sit. Were the fediverse to grow by an order of magnitude, I think it would need a reputation system to add a bit of friction to a brand new server or user getting a lot of reach quickly.