100% agree. But IMO mentality is super important in Sport and is IMO the defining difference at the top of the game (whatever it happens to be). BUT this defining "mentality" isn't seen by us, or the press, it's rarely witnessed by anyone outside of the environment of the individual. The mentality is in the application of improvement which is required for training, to be obsessed with the detail of a freekick the ability to repeat a technical aspect of their control. To self analyse and repeat this over and over again, day after day, week after week. It's a kind of OCD (it might even be OCD with some, I've heard it said of Beckham, Gazza and the Ruby player Jonny Wilkinson who would all set a target of doing something 'X' times and if any of them failed, they would start over until they completed it, sometimes this would go on for hours).
What is nonsense is if someone who claims that winning mentality is someone who only thinks of winning and therefore that makes them a winner.
Winning is the result of the application in improvements. It's going into a competition confident that you know that no one trained harder. Fear is a big inhibitor in sport, it manifests in the minds of professionals who know that they haven't trained as hard as their opposite number or opponent, this is when someone is already beaten before they take to the field.
It's also why I think there was a lot of PR surrounding Poch's (and Bielsa's training regimes), rest assured other teams also train just as hard, but having the players believe that they have trained harder maybe the 1% marginal gain required (in a team multiply 1% x 11 players and that's the difference between winning and losing).
The above is not just about being fitter, or believing you are fitter, it also applies to technical ability too. Technical players all work really hard on improving their technical ability, but if they believe in their technical ability, then they are more than likely to feel confident to use it. This is amplified when they play for a coach who helps facilitate the technical ability of their players.
But I stress, none of this is seen and if it has been it's still not quantified, watching a team or an individual train without seeing the opposition results in bullshit analysis, but if the Clubs make a thing about it like they have with Beisla and Poch, it means it gets reinforced by the media which helps, even more, the player buy-in.