If I burn my hand on a stove, am I obligated to to do a blinded A/B test with N=15 before I decide, for my own personal case, that the hot stove caused the burn mark?
I don't get why you're putting people on trial for noticing their own individual reactions to food. The article itself admits some people are sensitive to MSG.
I'm going to answer an anecdote with an anecdote here, but I was getting headaches that were completely correlated with food containing MSG. I "researched it" (cursorily google searched) and found that MSG was totally safe so I didn't want to believe it, but the correlation was there. Later I found out I had high blood pressure—the high MSG foods were also high sodium and the sodium was kicking my blood pressure up really high and causing headaches. When I got my blood pressure under control I tried MSG again and no headaches at all.
The body is complicated and things aren't always what they seem to be to our pattern seeking brains.