Chrestus was a common slave name meaning “useful.” Couldn’t Suetonius be describing an unknown agitator in Rome, especially since he writes as if the man were present?

That is a real possibility, and the identification is debated. The case for reading it as Christ is that Chrestus was a common pagan misspelling of Christus (Tertullian complains about it), and that Acts 18:2 independently records Claudius expelling Jews from Rome, which fits disturbances between Jews and Jewish Christians over the Messiah. If the skeptic is right, one minor supporting source drops out and nothing else changes. Suetonius is included as a caution, not a pillar.