I think (not totally sure) the point is that it's a political trial, not a criminal or even really legal one. It's a different standard. Sure, he didn't literally say the words 'go and kill Mike Pence and any other senators you can get your hands on, I support you', but what he did say is clearly bad enough for any sane senator to consider incitement.
Also, there's the political capital of forcing Republican senators to either vote for or against impeachment - which is lose lose - and the precedent that acts of this kind will not be tolerated - which, while just semantic, is surely an important statement to make.
And also - it was this time last week that a tape was released of the president spending an hour attempting to threaten and coerce a secretary of state to overturn legitimate election results in his favour. It's all been more or less forgotten about, but that in itself is utterly outrageous and certainly impeachable.