It depends on the amount of third-party crates, unsafe{} and assembly being used to confirm this claim. It's possible that out of the other alternative AV1 encoders (not in Rust), this claim may or may not be true, but can be more true if they said "The fastest and safest AV1 encoder in Rust".

Most rust projects have these sort of safety claims to attract companies, devs to promote it in meetups or tech conferences. Maybe they should audit this encoder to see if it lives up to these claims.