Stuff

Time to bring you some fact then, to hopefully convince you, or you can just play the game again and test:

First off, here's a picture of the hitboxes: http://imgur.com/a/Wh6rR. T
hese you can also see in action in these two videos https://www.youtube.com/watch?v=eDaYK1cOCmw and https://www.youtube.com/watch?v=hoUmUs9PeI0

This was done at a live charity event this January, where you can see the runners jump through enemies in the same fashion as seen here: https://youtu.be/w30Pbe-B0lo?t=1257

And the former WR holder, Andrew, also played in front of Shigeru Miyamoto, terrible run, sadly, but you can still see him jump through piranha plants: https://www.youtube.com/watch?v=7mEyjLpLAKM

This page has a lot of general information about speedruns for SMB: https://kb.speeddemosarchive.com/Super_Mario_Bros.
And the WR holder darbian made a detailed tutorial explaining many aspects about the game and the run: https://www.youtube.com/watch?v=PInZ2-qfKEU

Or you can, as mentioned, turn on the game, if you still have it around, and test some of these things to be true. There is an endless amount of basic evidence about how this game works. It's only up to the ignorant to care or not :).
Lesson out.