DUMBASSIAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA!
The problem with Solo Q is that once they engage on an enemy they refuse to switch focus until their first target is dead.
But..... he's Garen
I think it's fine if they are the only thing in range, then it's better to hit them then hit nothing at least. But there is also the difference between that, and as has been said, focusing the mundo that just ulted when their is say, a vayne with 500 health right next to him.