It shows a strong mentality in that game - but it doesn't change the fact that the FA Youth Cup is by far, a more important game. A lot of the players who got their chance with Uniteds first-team (or at other clubs at the highest level), had one thing in common - they performed in the FA Youth Cup. Blackmore, Hughes, Whiteside, Class of 92, Lingard, Pogba, Keanex2, Greenwood, Mainoo, Garnacho, Welbeck, Drinkwater all either won, reached the final (Welbeck and Drinkwater) or performed individually (Greenwood) really well in the FA Youth Cup
In addition I can think of 5-6 other players who made a good career for United in the last 20 years or so where I don't know how they performed in the FA Youth Cup (McTominay, Evans, Cleverley to name some), and some who were part of an FA Youth Cup-run that ended quickly (O'Shea being one) - but the general rule is that a good performance in the FA Youth Cup at best increases your chance of making it at United - quite a lot.
Of course it also has to do with qualities - the general reason is that players who don't perform in the FA Youth Cup simply aren't good enough - and that's why they never get a chance at the highest level. But when you have a team with 11 straight wins in the league, despite changing 3-4 players every week - they should be good enough in terms of footballing qualities. At least some of the players should be good enough to have a chance at making it. But the first time they play a really important game - they fold. Against a team that is 2 levels below what they play every week. Not a single player showed any desire or any will to win.
I can promise you - what I wrote here, every single player in that game got from Lawrence - in a much harsher tone. "Do you have the will and desire to make it ?" Some of those players might get a few chances to prove they have what it takes - and they need to take it when they get that chance. And doing well in the FA Youth Cup is the easiest way to prove that they at least deserve the chance - not all of those players will get that chance because of that defeat against Swindon.
So yeah - I noticed your mocking tone. But I stand by my words - if a team as good as that, fail the first time they play in a match where the result is more important than the performance (although both was awful in that specific game) - it says a bit about their mentality.