ugh....not to get into the gordian fucking knots of debating how the force works again, but when I think of this, I always try to imagine what Luke actually experienced.
Because it's an interesting thought experiment. Snoke, when he was mind reading Kylo Ren, was crazy specific regarding what his feelings were. The wording of the 'He strikes down his TRUE enemy' is particularly interesting because Rey never deceived Kylo Ren or anything like that, so applying 'true enemy' to her like she was hiding something nefarious, when he himself knows she never did, meant that he was reading Kyo Ren's true feelings...but he had no idea that the target of the feelings he was reading were him. The only way that wording really makes sense is in the framework of Kylo Ren realizing, in that moment, who his truly impeding his desire for progress.
This suggests that rather than the literal things that the person being read thinks, force mind reading is more abstract. He can get the exact feelings and emotions of what happens, but not the literal thing. And this makes sense with the OT. The Force, in the cave, showed Luke himself being under the mask, communicating that the inner feelings Luke experiences upon finding out his father is vader. Luke now sees himself under vader metaphorically, and that's what the force shows, a metaphor, and it does it through feelings.
So, when Luke took out his lightsaber, I don't think he was actually in his right mind. The force sent him a vision and he experienced it not just as a ghost that happens to be at the scene of Kylo Ren slaughtering people, but feeling everything that scene itself entailed. As a result, it's pretty reasonable that Luke taking out the lightsaber would be acting in the same sense as if the vision had already happened, atleast emotionally, so it takes him a bit to re-orient himself to the present, where Kylo Ren hasn't done anything yet.
