modding on xbox and on pc are two different things, so i doubt xbox tos means anything for a pc game.
on xbox modding usually requires you to mod your xbox first, which itself is against their tos. most of the times when you mod a console, you unlock it for online cheating and piracy, and they really don't like either. doesn't help that when people think of mods on a console, it's almost always for online cheating/hacking.
on a pc modding is usually more true to the title, since most pc game mods are actual game modifications to add or change content, not just an invincibility hack that disrupts online play and affects other players' opinions of the game and the company that owns it.
it also mentions single player, probably because of its requirement of connecting to mojang/future microsoft servers. it would mean most likely every client and server having to run in offline mode, so no more skins, no uuids and such. or alternatively a custom host for those.