Your body is not the specific….you can get just as much of a training benefit from being on a spin bike as your regular bike on a trainer.
As for fit, I have a pretty long and low position and I can come pretty close to replicating it on most spin bikes.
However, as noted by others, we are not the primary target for Peloton. Their main demographic are exercise enthusiasts who will use a spin bike as their tool of choice. They are not targeting cycling enthusiasts. There is not a lot of overlap in that Venn diagram.