I have seen players send off for "less" than that (Romero's tackle)
Do you believe Haaland is rattled each time he gets tackled hard in a game and then "turns off"? Haaland's been tackled hard many times before Romero's and still scored goals. If that were the case then every team would do that to Haaland from now on and he stop scoring goals.
The whole City team underperformed in this case, they simply didn't manage to supply Haaland with the necessary ammunition, and that not down to just that tackle.