MSG is the salt form, wherre the glutamate is bound to a sodium atom. In food, my understanding is that MSG will split into two things: sodium ion and glutamate ion. The difference between adding MSG to food and food being already high in glutamate would be the salt content.

I don't recommend telling people their subjective experience isn't true- you don't know for sure that they don't actually get migraines from MSG. I think it's fine to tell people that often their subjective experiences can be colored by prior knowledge, and people often ascribe causes to unrelated factors. (My personal belief is that most people who say they got a headache from MSG experienced a headache, but consuming glutamate was not the cause).