I can push mobs into blocks and they start losing health...

It's called suffocation damage.

http://minecraft.gamepedia.com/Damage#Suffocation

i think he means that he can just nudge them into solid blocks by himself. rather than them colliding with the blocks,

So i was testing this and it turns out that only mobs riding a Chicken or the thing being ridden is a baby, Mobs that ride start to suffocate inside the wall as the mob being ridden doesn't