Actually the GP is incorrect. It's pretty well understood [1] these days that your body is like a "cup" that can only take a certain amount of stress before it starts getting counter productive. Mental stress and center-nervous-system stress (what weight training does) both contribute so if you already have a lot of one you're forced to have less than the other or everything suffers.
[1] I'm not in a position to hunt links but lookup "Alan Thrall stress" on youtube.
[1] I'm not in a position to hunt links but lookup "Alan Thrall stress" on youtube.