I guess I'm learning frailing on a five-string?

Yup, or at least that's what the Seeger book teaches. That's also what Mason's doing in the video. (Some people distinguish between "clawhammer" and "frailing", but they're basically the same thing. I'd have to listen again to see if Mason's doing any drop-thumbing in the vid, but he does do it a lot.)

A thing to try -- Mason gets a less ringy, more plunky sound that he likes better by using a folded-up sock or washcloth as a mute. He lightly wedges it between the (drum) head and the wooden bar inside the body, underneath the bridge.

Thanks for the compliment and happy frailing!