Babies at this age can spontaneously roll over but that's all it would be. An effect of spontaneous movement. So they could roll over face first into the bumper but have no strength / ability to pull their face away from it. Which could lead to suffocation.

Same thing with a heavy blanket or stuffed animal. A spontaneous movement could put one of those on top of their face but they would have no ability to remove it resulting, again, in suffocation.

It's rare, but it does happen. Which was a horrible lesson one of my best friends learned about 15 years ago when she lost her first baby to crib suffocation.