I've heard this frequently and forever -- the idea that some amount of power vanishes into frame flex, and a stiffer frame would put more energy into the rear wheel. The energy used to flex the frame, though, is released when the frame unflexes -- it doesn't just disappear or get turned into heat or sound. And as the frame unflexes, it actually applies force to the drivetrain. GCN did a feature where they demonstrated that a frame unflexing drives the chain forward, spinning the rear wheel.

It's worth checking out: https://thebicycleacademy.org/blogs/...lex-gcn-tech-1