on singleplayer mc serverside and client side is the same thing. and its really bad form to use a modified client when speedrunning. also should be noted that fabric does alot of changes with the way the game runs and he should have been disqualified in the first place.

Not to be a know-it-all, but "server-side" and "client-side" distinctions are still important even in singleplayer minecraft, because any singleplayer world you are playing on still has an "internal server" that works almost identically to the way a multiplayer server would, with a few exceptions. Thus you can have "client-side" mods that do absolutely nothing to how the "server-side" code behaves, even in singleplayer.

Also, from the sound of it you may not know this, but fabric mods are actually pretty widely used by minecraft speedrunners- there is a list of allowed mods that you can use during speedruns, which have been vetted by the speedrun mod team. People mainly use them for performance improvements, as well as some very minor quality of life changes.

Fabric mods do not inherently modify how the game behaves in drastic ways, because of the way that the fabric modloader was designed. The fabric modloader makes use of "mixins", which are essentially a way of dynamically injecting mod code into minecraft itself, without needing to modify any mojang code (unless you explicitly do this because it is necessary for your mod to work).

Dream's true error here is that he did speedruns whilst he still had his "recording mod" installed. If he were smart about it, he would have properly set up his .minecraft folder / game launcher so that the mods from different sessions don't get mixed up, but I guess he's just too lazy? Even though minecraft is literally his job? Lol. The part that really pisses me off is he just adamantly refused that he cheated for so many months, even though he knew all along this was the reason his drop rates were modified. Not to mention just... Everything about how he handled this situation. Incredibly immature.