I think the biting the comforter where you were is more of an action of trying to move, change, and/or smooth out the “lump” in the bed. As your bunny’s way to make it easier for him to run about. I know that my bunny Forrest (passed away many many years ago) loved to dig, push, tuck and bite on blankets, and it would get him excited and he’d binky around.
Jack, even though he’s much older now and does less of that, still enjoys playing. I recently discovered, when I accidentally dropped and left a few sheets of papers towels on the floor, that he loves to nibble, pull them up with his teeth, push them, and then binky. So then I got a about 6 connected sheets and set them up (sort of standing up in a way) on the ground, and he just had a good time — sort of like what you are describing with what your bunny does with the comforter.
I think it is a form of play. Animals sort of play by practicing what comes naturally. Like dogs practice their predator instincts by chasing a ball, rabbits can play by practicing their prey instincts — digging, binkying (which can also be an escape and fight tactic in the wild). Also when Jack would be naughty, he would then binky away. I always thought it was part of his prey instincts to “escape” from trouble. He just was having fun doing it as a house rabbit.
Sounds like play!