This was a fun watch, even though I don't think it would be worth playing. I don't know anything about making games, what engine would one use to do stuff like this?
Yeah, what's nice is he does tweak the levels a little bit too to make it feel more like the original (even though it's in 3D). Like some enemies follow you, or bullet bills appearing from the clouds below (to avoid adding new cannons).
At the end you really start to understand how differently you need to approach 2D vs 3D platform design.