explaining the joke
a 120-cell is the four-dimensional analog of a dodecahedron. like how a dodecahedron is made of twelve pentagons, a 120-cell is made of 120 dodecahedra
3D mario games are one dimension higher than 2D mario games like super mario world
thus, go one dimension higher than a dodecahedron and add the extra h from the speedrunner's moniker and you get 120chell