I don't agree.
There were times during the McFred era when he was caught out in a one man midfield covering for Fred going on his jollies halfway up the pitch. But even Casemiro can look out of position when he has no help from his teammates (as evidenced by yesterday).
Generally speaking, though, McTominay's positioning is fine. He's a difficult player to get past and even more difficult to dominate physically.
His passing is the most unexpansive in the squad, though, I'll give you that. In that respect, you can't expect him to do the DM job of dictating play from deep. But that's where being in a midfield 3 should hopefully help. If we can figure out the Bruno/Mount dynamic, McTominay should be fine as a backup.