Did his knees collapse? If my butt touched cold porcelain I would just stand up and put the seat down. How did he get sucked into the damn thing? Was this a vacuum toilet on the space station?
If you fall in, your butt doesn't hit cold porcelain any more than your foot would hit a missing stair. It just falls through the unexpected space, and if you can't catch with your hands quick enough it lands in the water. Sitting is already falling backwards, far as your balance is concerned. Especially in a familiar seat, you're not poised to stand right up again mid-sit. Your center of weight moves behind your heels, and it's difficult to stop. Your knees alone won't save you, you have to pull your upper body forward, and on the toilet there's often nothing in front of you to grab. Add in being half-asleep, in the dark, and it's quite a rude awakening.
How wide are your toilets? Even if i lift the seat up and sit directly on the porcelain i wouldn’t enter the bowl, i’d sit on the rim and I’m not a particularly wide individual. Also if your knees can’t stop you mid sit then you should work on leg strength.
Oh I could definitely fall in ours and I have quite a fat ass, not sexy fat I'm just fat. Also it's not strength, it's balance. I lift my husband from bed to wheelchair etc daily, my legs are plenty strong. That's also why he never leaves the seat up. If I were to fall in, it'd be my own fault. I'm going to try to find a link to my toilet, it's pretty new. If I can, I'll edit.
Edit: Here ya go. It's a very good toilet, hardly any splash, everything just goes down.