If people want to do something silly like brag about their mechanics on discord it's no skin off my back. I don't see why you care overly much about that as it is a purely ooc channel. I can see it being grating though. I don't see how it correlates to their pc's not roleplaying certain aspects of their character such as fear of death etc.
I never discuss anything but vague stuff with regard to my characters on discord. Not fond of discussing too much about their character sheet, but I guess some people like doing that. I certainly would try to avoid talking too much about the characters ic motivations or actions for fear of spoiling the opportunity to do so in game. So I don't think seeing what people say in discord is a good way to judge attitudes.
At least if it's only people bragging about something silly like mechanics in a game of fairly simple math. If people are being jerks thats another story altogether!
I really think you should get these six people to come forward and discuss their concerns in a more specific way. Having people who agree with you is meaningless if they don't represent themselves. I'm not saying that to invalidate yours or their opinions. I just feel that they should speak for themselves or bringing them up at all comes across as the "winner mentality" that spawned your initial post.
Fairly sure that isn't how you meant it, but its very much how it can be perceived. I think often people perceive things as a winner mentality, where there are just slips of the tongue, well intended, but poorly chosen words or actions spoken/taken. I'd also like to add for clarity's sake that this winner mentality extends to every aspect of the game. Not just mechanics.
At the end of the day the best way to beat the winner mentality is to try to focus on having as much fun losing as possible. Even when you're "winning" try to focus on facilitating ways that your opponents can beat you. It's surprisingly fun to throw your opponents rope to wrap around your neck. Sure you'll find there are people who will want more and more while giving nothing. But if you have fun losing you always win. Not to mention people are much less likely to risk murdering you if they think they can cripple you in a much less risky/permanent(thanks raise dead..!) manner!