Problem is, it’s up to the courts to decide what is and isn’t “official us business”. As of now, everything Trump does will be “official”, and nothing Biden does would be “official”.
Doesn’t matter what is fair and even cause Republicans don’t play by that game.
The Supreme Court isn't really interested in arguments, it seems. They're starting at conclusions and working backwards. In a sane world, you're probably right with the logic. But in a sane world we wouldn't have made it to this point to begin with.
It's also hard to argue that any "official" act a president does is immune from prosecution. There is absolutely no way to twist the Constitution's words to have that meaning. But they did it anyway.