I noticed he had a naturally shaky hand while he was holding the device and and pointing out pieces of it. We can't really deduce the force required just based on his hand shaking but either way this is a pointless test that really doesn't tell us anything new.He wanted to prove those people that didn't believe him. If you watch the video closely, he shakes his hand quite a bit to try to bend it, which tells me it requires a bit of pressure. It's pretty simple, don't put the phone in your back pocket and don't sit on your phone.
The phone is made out of what is essentially an aluminum can, it is thinner in some places and thus weaker, we logically can't expect it to hold up much better given he makes a point to apply pressure on the weakest points.