Johnson wasn't just impeached for trying to fire some Cabinet member. He undermined the rule of law by purging the govt of defense officials who were trying to fight white supremacist terrorism.
Katyal, like Turley, argues that while Johnson's actions were illegal, they didn't rise to a "high crime" — that is, they didn't fundamentally undermine the govt.
They refused to see his actions through that context. They saw it as just regular lawbreaking, when in fact he was unlocking the back door for a white supremacist coup.