Where does it say? :(
I must have missed it, sorry if I asked a random annoying question.
But I don't understand, everyone has to run /sideload to get any mod for online coop, but online coop is already a mod, isn't it? It's confusing...
In chat, when you try to do /gml, it says "Sideloading is not enabled.", and then NTT-FAQ has a "sideloading" item under modding commands explaining what this is.
The thing is, there is online coop itself (which remains fairly faithful to how the base game is intended to play), and there are numerous mods that NTT can load. The later can change how the game plays (be it by introducing additional content, or adjusting how the game functions - including things like framerate/timescale/viewport size). Doing the command for aspect ratio/framerate also counts as a mod - it's just that underlying functionality is integrated enough that it fits into a single line of code that you paste into chat.