At around 5:30 while climbing past the Boulder in the snow mountain he gets hit by the Boulder, animation and sound both play verifying the damage (and its just obvious it hits him) but he doesn't lose any life???

What mechanic causes this and is there a way to replicate it?

Dont you only lose a heart every second hit?? I could be wrong but i seem to recall this

As mentioned above, on easy you lose 1 heart in 2 hits. There's also some minor differences like cakes and invulnerability coins on easy.

Oh I didn't realize the runs were done on easy, that makes sense