No...all finger. The "Hammer Stroke"
I remember reading long ago Tim discussing how he 'reinforces' his index finger hits with his thumb resting against his index finger (Tim can you find where you wrote about this a few years ago?) ...especially when hitting multiple notes in a row with his index finger. This makes sense as a technique and may in fact be mentioned in one of the tutor instruction books. But I'd also like to mention that not everyone does this. Myself for instance (not that I'm any great example to follow) I always keep my index 'hitting finger' on its own, unsupported by the thumb when striking a note. Not everyone holds or uses their fingers exactly the same way, especially since our hands are physically all slightly unique.
I don't know if this is what Tim does or not but your description (Strum) sounds like what I do when/if I back up someone on guitar. I hold my thumb against my index finger as if I were holding a pick but instead, use my finger nail, down only....Alternating bass note with a chord.
Converse, in 1886, claimed that there are but 2 movements to Stroke Style banjo...the Hammer and the Combination. I have found this to be a truth 99% of the time.