> Does anyone agree with this?
Nope. Even millions of years ago when I kept dogs, and didn't have the nutritional knowledge that I now have, I never had behaviour problems. I do now try to go for more natural completes, and don't purposely buy food high in sugar or colours, but I have (recently) mixed a very cheap food with my decent stuff, as someone gifted me a 15kg sack. My dogs didn't leave home or make me walk the gang plank.

I have always joined obedience classes, since the 70's (with Toy Poodles then, cause I couldn't have a Collie at the time.) It wasn't any more difficult to train then, as it is now. So, with all the assortment of breeds I've kept over the years, I can't honestly say that food makes a huge difference.
I realise that some will say that the 'E' numbers to blame, so I emphasise that this is my own experience.