There was a study done on this, I forget where I read it, but it was about how light affects how people act in tense situations. The short version is that darkness makes people feel hidden and bolder, and sudden bright light breaks that spell because now they feel exposed. A headlight in someone's face does the same thing a phone camera does, it flips the power in the room. You basically showed that guy he was being seen, even if nobody else was watching. Smarter than you're giving yourself credit for.

Did the GC ever ask what the thinset maker says on the bag, or did he just skip that part? That 58 degree overnight temp is the whole ballgame too, since most thinset specs want 50 to 70 degrees minimum for 24 hours, and cold slows the cure a ton. For what it's worth, you can't really "talk" a guy like that into caring, so next time maybe put the cure window and temps in writing with your bid. He gets to see you're not guessing, and if he still waves you off, that's his cracked grout six months later, not yours.