Rule number one, dont listen to the guy just because he is big. These sound like bb types who do not know their ass from their elbow.
Warren Sapp was a hell of an athlete, doesnt mean I would want him to coach me in the weight room or on the football field, because regardless of his individual talent, he is an idiot and a maniac.
The wider your stance is, the more knee flexion and hip mobility will be required.
Its the exact same as a squat, sumo stances are not bad for your knees, doin it wrong is.
If the hips allow the knees to track out over the feet and dont show valgus collapse.
I personally prefer semi-sumo with a straight bar and teach that first to all of my clients.Pulling super wide sumo all the time might lead to some hip issues.
I would suggest to go with your gut when it comes to guys like this. From the comments in your paren. it would appear you already know these guys say alot of stupid shit, right?