This play is traveling.
This case plays details what the player may do.
4.44.5 SITUATION B: A1 dives for a loose ball and slides after gaining control. A1 is in a position either on his/her back or stomach. What can A1 do without violating? RULING: A1 may pass, shoot, start a dribble or request a time-out. Once A1 has the ball and is no longer sliding, he/she may not roll over. If flat on his/her back, A1 may sit up without violating. Any attempt to get to the feet is traveling unless A1 is dribbling. It is also traveling if A1 puts the ball on the floor, then rises and is first to touch the ball. (4-44-5b)
The case says they may sit up if they're on their back. It doesn't say they can rise to their knees if they're on their belly. If that were to be legal, it would be listed. Further, the travel rule is one that is based on listing what is legal and what is not specified as legal isn't.