This is not the case with the T91. I can’t speak for other piston AR’s, but I would assume they’re all alike in the sense that the BCG just has a strike face on it instead of a cup. The piston just takes the place of the gas tube and physically pushes the BCG rearward, but the buffer and spring are still required to put the BCG back into battery.
Now AK’s are a different story… The return spring is all contained in the back of the BCG and receiver, so the stock can be folded or absent. Some AK’s can be fitted with a buffer tube and AR style buttstock, but this is purely aesthetic and preference, and is not required for function.
Hope this helps!