No, only you read it that way because you're such hopeless pony fanboi ;) Just joking.
I wrote it because mustang was also heavy on controls, not as much as 109, but still heavy, especially at higher speeds. Believe it or not.
Now this is what i call exaggeration. It was heavy, but fully manouverable at 600kph. None of the ww2 pilots were exactly body builders and they had no problem to control it even much higher speed that that. Stifness was only real problem when you entered deep, long dive and speed increased well over 750km/h.
I have read that and dozens on finnish and german 109 pilots reports and diaries.