FWIW, I get a seriously intense headache precisely 9 seconds after eating a large pinch of MSG (because it’s so tasty). It completely goes away 10 seconds later. I have no doubt it can cause headaches (in at least some people), and everyone denying it looks ridiculously sanctimonious to me.
I bet there are some self-delusions about these headaches, but they’re not 100% fake. You’re being just as foolish denying them as the people who are 100% sure they’re all real. Reactions can differ from person to person.