Its how i lift too, peeps on here seem to have big problem with it but i dont see the issue, if your shoulders are pinned back at the start then they arent engaged during the lift they only perform a structural support role. I personally think this will increase the point load on the lower back.
But hey guys who lift 300lbs love telling guys who lift 500 lbs that they are doing it wrong dont they?
Just because someone lifts less, it doesn't make them any less of an authority to speak about form., Many coaches, world class olympic trainers don't all have the ability or talent of those they train. Remember that.
It's about sharing and ensuring best practices are shared - it's almost a duty of care amongst powerlifters/weightlifters/vanity lifters (bodybuilders

). If we don't ensure we help other people we're being disingenuous to the sport and to those around us.
Striving to enhance technique is FAR more important than the weight you lift IMO.
I'd rather be able to lift a lower weight, perfectly several times, than a heavy weight once badly. I get a lot more out of my exercise if I do it properly - and if it means I've had to take a step back, well so be it. I honestly feel stronger than I have ever been despite not being able to shift the same numbers (I'm only 10% down really).
As said, that weight is heavy, and if he doesn't have the strength or capacity to hold that weight any better, considering the rest of his form is reasonable, other than his lock out which is atrocious, his back will round - however, as someone who is supposed to be a "leader" one should lead by example and lift correctly, rather than "just get that ****ing bar up" mentality. Sure in a comp, it's different you're not coaching or sharing best practices - however one would hope that keeping the core principles of lifting that form would be reasonably good throughout - I accept at 1rM or as you get towards it there will be that 10degree variance which you must accept.