Some of the levels he made were conceptually flawed but the dude is not an expert level designer nor did he ever claim to be. Also the commentary in the video really does explain that he is aware of some of the problems like not being able to get the fire flower to fall down. I thought it was a good level as well.
Level could have been made easier with 2 additions. One is warp pipes that take you to the sub world then back to the main world in the cages so if you messed up you would not have to kill yourself and everything would reset. That would solve the big issue of havingto kill yourself.
If he used the Super Mario Bros 3 theme he could have made fire flowers super leafs so they gently fall to the ground solving that issue. He would have to design aspects of the make yourself small mechanic there but they wouldnt be difficult changes.
Most of his levels suffere from the lack of checkpoints in the game. I think the actual challenge and uniqueness of the levels will make people still wanna play through them though.
Just wanna be clear, and not sure if I was, but--everything you said in your post, I 100% agree with. It's pretty much what I was trying to say in mine.