Any decent player who belongs in a successful team would think 'I'm going to prove him wrong and get in this team'. Look at Schweinsteiger and even Fellaini at United, written off by some managers only to then get back in favour. If a player is sulking then they are just confirming Koemans initial judgement.
I agree with the theory, absolutely. As I said, I'm not saying it's necessarily a bad thing, and we don't even know who he was talking about.
It's not always that simple though. Footballers are humans at the end of the day, they have feelings, they can have a crisis of confidence and they don't necessarily get better through trying harder. It's also worth noting that they might not even
want to prove him wrong. Sometimes a player might agree and think maybe it's time for a change of scene, so they switch off a bit while concentrating on the next step. Maybe they need to be told they're wanted, need an arm round the shoulder, or thrive off responsibility. If the manager had said that to them instead of saying, 'I don't really fancy you' maybe they'd have gone from strength to strength.
Fellaini's an interesting example to use- a player many Everton fans demonised as the archetypal bottler and sulk now being lauded as the example that the current players being demonised as bottlers and sulks need to aspire to.