It’s a bit of a rigid thought process that starting on the flank means scoring goals is automatically going to be harder. There’s all kinds of front three systems where scoring goals as a wide forward has actually looked easier. The modern game is dynamic enough for CF’s and LW’s to interchange positions.
Auba’s as service reliant as they come and is a master at getting into the space that the defender can’t cover. His back stick poaching is one of the best in the world and that accounts for most of his goals. He is equally prolific starting from any of the front three positions because he’s effectively still going to be at the end of chances in central positions.
This theory that playing the two up top together is going to optimise their threat more than now is kind of based on 4-4-2 era idealism more than practical end results.