I was not exclusively refering to this chapter. There was a good part of last chapter where mc and the traitor were just standing side by side talking. Turning your back to an enemy is such a galaxy brain maneuver.
Well, if you really wanna get nitpicky about it, the enemy NOT doing anything to the hero while he is busy turning his back on her is also a pretty big galaxy brain maneuver.
In this case though, it's obvious the maid girl has other plans, and it doesn't include killing these people immediately. And our hero attacking the maid was not going to help the situation in any way. The maid is basically a non combatant during this fight, and the real problem is trying to wake the princess before her persona steals all her life force, so it makes sense for him to concentrate on doing just that.