It was a cleartext signature, not a detached signature.
Edit: even better. It was both. There is a signature type confusion attack going on here. I still didn't watch the entire thing, but it seems that unlike gpg, they do have to specify --cleartext explicitly for Sequoia, so there is no confusion going on that case.
I haven't seen those outside of old mailing list archives. Everyone uses detached signatures nowadays, e.g. PGP/MIME for emails.