Good question and valid point!
I would answer that "eye test and basic logic".
It is as clear as day that in modern game fullbacks have to be explosive, strong and fast.
None of those attributes are among Archie main advantages. Fast wingers in EPL level at least can just get past him with first three steps.
But he is good at reading the game and situations. Having seen tens of his games for us - he was so clearly more comfortable in the middle (meaning in central defense compared to fullback).
And the last thing - no one would pay 40 mil for unproven fullbacks. You just don't. In that position this 40-50 mil is price for internationally established fullback.
If he truly was purchased as fullback with that kind of a transfer, it would be total fuckup, sorry
. I refuse to believe that even our recruitment could do something so stupid
.