The school clearly knows they can't go after him for the racial slurs because he was on camera that whole time, so they chose not to focus on that. But it doesn't mean it didn't happen.
It wouldn’t be strong evidence either way, OTOH, typing is probably fairly limited usually in Zoom classes and usually associated with other in-band indicators (like chat messages in the class); it's pretty likely that the feed of a student on cam and described as doing nothing unusual would have pretty strong evidence that they weren’t doing what he was accused of, even though if he was guilty and on cam you’d expect only weak confirming evidence.
I think you're wrong about that. I can think of about a thousand reasons a kid might be typing instead of paying attention to the class. This hypothetical evidence is too weak to damn the kid, and the only reason it would even be considered at all is because all of the other evidence is weak as well. The whole thing is a farce.
The kid not appearing to do anything else, not having light from opening new windows flashing on his face, etc, would make me far less likely to think it was him.
Source: Have been teaching remote middle schoolers this entire year, and it's been pretty easy to spot a pretty big fraction of those kids that are opening other windows, playing Fortnite, etc.