I think your confusion is warranted, because it's not clear how SCOTUS' decision is different from what the Constitution comes right out and says. On the surface, it does seem to just reaffirm what we already know, and maybe the liberal justices are just whinging.
The trick is that they did it in a way that causes a lot more work in the courts. In turn, that means Trump's trials get delayed further.
Nobody sane is going to argue that getting a hostile crowd to surround and storm the capitol while an important procedural vote is taking place is an official act of a President. But now it has to be ruled on, specifically, and that's one more thing to add to the pile before the obvious verdict can be reached.
Trump's lawyers have already filed an argument in the hush money case that certain points of evidence should be removed because they were official acts. If so, that would potentially result in a mistrial, and so the only Trump criminal case that went forward would have to be redone.