I'm not a top fuel expert but at the risk of posting the wrong answer:
I believe the cubic inch limit for Top Fuel is 500 CID. Not sure if it was in the sixties, but the wide bore centers of the Hemi and stroke capacity make that limit easily. The Hemi is a robust design that seems well suited for the double whammy of nitromethane and boost.
I think your friends are correct.
IMO, the hemi chamber with centrally located sparkplug(s) evenly distributes the enormous combustion pressure on the piston dome. With a wedge chamber the flame travel goes across the piston.
Other engines have been used for Top fuel, it's just that the Hemi was more successful in the long run, so more racers chose the Hemi. As the class and records got higher, more and more specialized parts had to be developed to keep pace. By the time Top Fuel progressed into this century, Darwin's law weeded out the rest.
There probably isn't many (if any) stock parts on a Top Fuel engine anymore.
Top Fuel is very expensive and limited to a handful of racers, so it's not cost effective to develop parts for other engines...that makes the Hemi the only practical choice these days.