I mean, was EVERYONE including Gohan nerfed to 4-B? Because I don't recall Piccolo or Gohan stating their power was weakened, which would imply Gohan is at least Pre Tournament of Power SS2 Goku level at Max power, considering he hasn't finished his training with Piccolo. If this takes place after BoG, RoF, and the Goku Black arc, Goku, Frieza, and Vegeta should still be Universal, if not Multi-Galaxy+ or something? I don't think that the plot makes it clear that they were nerfed to Buu saga levels specifically, that's just an assumption. All we know for sure is that Goku can't use Blue in the story. To say he can't use SS3 except for short bursts without any direct statements is also an assumption. Goku has gone in and out of SS3 before in videogames just to use a powerful attack. I really don't think it's a good idea to assume he can't Sustain SS3 if he wanted to based JUST on him being restricted to some unknown degree. Just because he's restricted, and uses SS3 in short bursts doesn't mean that he is restricted to using SS3 in short bursts specifically. There's plenty of Meta reasons why the game developers wouldn't give Goku all these forms and shit, and there's only a THEORY suggesting Goku is restricted to Buu Saga levels specifically and not anything else.
So, I think that, to be safe, we should do "At least Solar System, possibly Universal" for 21's Majin form. Seeing as we don't know how much Goku is debuffed for unless Matt can provide scans specifically demonstrating that Goku and others are at Buu Saga levels (and a statement showing that Goku can't use SS3 for anything more in short bursts). I can see where you're getting that interpretation, but it's not exactly hard fact.