That bit is patently untrue...defensively decent and makes the odd mistake like every other player. Only those whose confirmation bias ignore anything positive in favour of this narrative keep espousing this view.
Tav's effectiveness in attack is precisely because he's coming from deeper positions, when the space opens up with everything in front of him, be that driving into the space, arriving late at the back post or coming in to be an extra body in central midfield. You simply don't get that if you become a winger. Entirely different kind of game, and the reason EVERY manager he's had at Rangers has recognised that it would be foolish to change it.