To answer your question: Yes, that can be true, but is doesn't have to be. As someone said, you don't train Ronaldo to do a fancy flick to then be able to a have free shot on goal. Of course, these things are intertwined on several levels but I think, you are clinging too much on this term "moments of brilliance".
When that term is used, it doesn't necessarily refer to brilliance or excellence or high quality. It is more in contrast to "a well worked goal where a team does a chain of actions to end up with a high scoring chance".
"Moments of brilliance" or "individual brilliance" are more meant to emphasize the "coincidence" or "fate" factor in a goal. It is tricky to describe and I think, the term is a bit misleading in itself but for me, the difference is this: when I sit in front of the TV and I feel the pressure is rising, somebody has overcome the press, now he's executing a through ball to release a winger in space without a defender in front of him and in the middle the striker has started to move into the box, I am getting tense, I can't sit relaxed but will go closer to the screen. This is what I would describe as a "well worked goal" where players took smart decisions and executed a plan. On the contrary is for example Greenwood dribbling close to a packed penalty box, shooting with a low scoring chance, the keeper can't get a hold and Ronaldo takes the rebound.
Individual brilliance isn't necessarily great play or a great piece of skill - it might be just making use of an opportunity resulting from a mistake. Of course no trainer will stop his player do that stuff. But if you are mostly reliant on scoring rebounds, deflections, 30yard screamer, direct freekicks, flying header, goal keeper blunder, VAR decisions (not saying we are relying on that right now), than at some point it will cause you some issues. Any team makes use of these things, they belong to the sport and are part of the highlight reels. And nobody wants United to stop scoring such goals. But, our opponents score other goals as well. Set pieces, tap-ins, third man runs and so on. I am not saying we don't do that occasionally from time to time but the feeling is, that while other teams rely on individual brilliance to 70%, we do to 90%. Which gives us some disadvantages at the end of the day.