That's actually a misconception within a misconception.
It's not that MSG allergies don't exist, it is that they are often downplayed for the same reason that Celiac's disease is downplayed. When a few people fake or overexaggerate their symptoms, credibility is taken from the rest of us who actually suffer from it. Now people are always quick to invalidate those who are symptomatic.
Yes, it's true that some of the rumors around MSG are racially motivated, and that some people who claim to be affected are lying. But that doesn't mean that MSG related symptoms aren't real for the rest of us. Speaking as someone who is from Hong Kong, grew up with MSG, and absolutely loves the taste of it, but developed health conditions that were comorbid with MSG intolerance.
As a chronic pain and migraine sufferer, large quantities of MSG is a common trigger for migraines (or headaches when I'm lucky). I've been blind tested before with someone else's help using the same quantities of salt vs MSG in a cellulose capsule. Each time, I would happen to be fine after taking the salt capsule with a glass of water. But after taking the MSG capsule with a glass of water, I would have have a headache or a painfully tense sensation around my head. This was done multiple times across separate days to rule out confounding factors.
It's likely true that for the vast majority of people MSG doesn't trigger a reaction. However, a few of us have an intolerance and we are frequently dismissed and medically gaslit. Please believe us. I'm so tired of people telling me that what I'm experiencing isn't real. I wish it didn't have to be real so I could go back to eating whatever I want and not worry about migraines.
Jokes on you. I got parentified early enough that I only had to watch my parents grow up and not myself.