Unlike the normal clip, this one takes place in the first room. Similar to 1-2 in smb1, clip into the 2 block high pipe at the end of the level. Saves you from having to preform the walljump.

it's swag but loses time if I'm properly understanding your description. basically, once you clip into the pipe and walk to the end, the piranha is still spawned in its upward motion and you gotta wait

Idk where you got the idea that doing a 1 frame jump at a precise subpixel to clip into a pipe is somehow easier than doing a 2 frame walljump.