Grossman is right, Gary Davis only use thumb and index.
Merle Travis also used thumb and index only (other fingers on the scratchplate).
Mississippi John Hurt used thumb, index and middle (ring and pinky resting on the guitar).
Skip James used thumb, index and middle - and (judging from this
https://www.youtube.com/watch?v=i-XOO6XeM8Q) with his wrist high above the bridge, similar to classical position, not resting on it as the above players did.
It may or may not be significant that both Davis and Travis used a thumbpick and neither MJH nor James did (again, only judging from these rare videos).
But there is no significance to how many fingers they used - other than to point out that you need
no more than thumb and index to play in Davis/Travis style. Everything they did can (obviously) be achieved with nothing more than that. But that doesn't mean that the rest of us have to limit ourselves in the same way, if we find it helps to add another finger or two!
Personally, I use thumb, middle and ring, with index as my spare when needed. I taught myself without benefit of any books or videos - just from records and finding what fingers worked best. My middle felt like my strongest finger, and the one that reached the strings more easily than the index from the right hand position I was using (mostly resting on the bridge, but not using a thumbpick). The ring was the natural extra finger when needed. The index had further to stretch to get to the strings, hence it became 3rd choice.
It was the same when I started playing classical pieces. I found I could play them all perfectly well using thumb and two (sometime three) fingers, but still with middle and ring being primary.