As such, there's a good chance this is just an unintended glitch and access will be revoked in the next 24 hours or so, but for now, PS Plus members do seem to have access to Final Fantasy XVI. It seems you need to trigger a download through a PS5 console because the web store doesn't flag any PS Plus inclusion. On the system, there are reports you can either stream the RPG or fully download it. Final Fantasy XVI hasn't had a PS Plus Premium trial, so the addition of PS Plus buttons to the PS Store listing is new.
While you do have (presumably limited) access via PS Plus, will you check the game out? Let us know in the comments below.