Bats have both a much more limited gut microbiome than we do, and their digestion only takes 15-30 minutes due to their small size, so there is not much time for fermentation and gas production to occur. While science hasn't ruled out that bats can fart, no such toot has ever been observed. They are generally considered fartless.

As for the stunning glutes, it looks to be mostly an illusion. They do have some muscle back there, of course, but it's mostly from their natural resting posture jutting out those femurs, and then being covered in fur. It would probably be disappointing to try to pinch a bat butt, compared to how it would appear.

Furry Bat Butt

Bat Butt Anatomy (warning: dissection image of bat leg, but not gory)