I can't speak to OpenBiome specifically, but you are correct, they look for generally well people. I'm active, but they don't expect me to spend 1hr in the gym everyday. There is also research showing that the microbiome of obese and overweight people differ from healthy weight people, I'm of the latter group.

Diet wasn't so much of a concern. I once switched to gluten free on request from the recipients family, however, as both myself and the physician said, gluten is probably broken down before it comes out the other end.