I'm not saying there isn't an adjustment, and as those articles say some guys have less of a problem than others, but it's not that unusual and certainly not something that most guys can't overcome. And Tyron Smith is a terrible example since he was an elite left tackle who was a 4 time All Pro and named to the NFL's all decade team. He may have struggled a bit early on in 2012(presumably when this article was written) but clearly it was the right move in the long run.
So if the argument is that Davis, despite saying he has no problem playing elsewhere, can't play left guard, it's a very flimsy one.