Yeah that's the clip I mean. There was a setup found for clipping into the top of the pipe rather than the top of the pipe entrance that does not lose time over the entrance clip. It is significant because the entrance clip is hard as shit.
Minus world on NES only has one level, but on the Famicom there are three minus levels, and the last one has Bowser's axe at the end. Touching it will bring up a princess is in another castle message, but it is considered to be the end of the game because you can't progress past that point. There is also an underwater flagpole glitch in minus world that skips the entire walking to the castle animation, which is pretty cool.
I think the only real time save left in Mario is a fast acceleration in 4-1 that has only been achieved by a human once, flagpole glitch in 8-1 and whatever time there is to be scraped from 8-4. 8-4 does not work on framerules since you don't need to wait for a level to load after it, so there is always time to save in that level, but I believe the current record has the fastest 8-4 there has been in a record run.
I have not seen this before, but it is similar to other strategies that are used in runs. There is something called fast 4-2, and it's difficult to explain why it is faster than other ways of completing the level, but a clip like this is used. It is not used to actually travel faster, it is used to push Mario to the right of the screen in order to abuse the way the warp pipes load exit screens.
I believe one of these clips is also used to skip a small section in minus world or some other smb1 category/variant. I have never actually seen ground clipping though.
Edit: these clips used to be considered TAS only and not humanly viable. The inputs to make them work are very specific and quite tricky. I think that the next world record will probably include the 4-2 clip (if it isn't in the current one, can't remember) purely because there is not a lot of time left to save.