Aspartame is also a trigger, but the fact that one person has multiple triggers doesn’t mean they are related at all.
Now you’re right that MSG is more than sodium. Sodium can be a headache trigger, including migraines. Glutamate is also a migraine trigger and a fairly common one. It doesn’t happen to be one for me. However, it is a neurotransmitter that is involved in pain signaling. It’s understandable how it could easily trigger a migraine or make the pain worse.
Some triggers for some people actually help other people with migraines, like caffeine. Migraines are such an incredibly complex topic that there are medical specialists for them. Mine can be fairly debilitating, but are rare enough I don’t qualify for most prescriptions. So I definitely understand how trigger management and symptom management are a big deal.
For me aspartame only just recently started giving me headaches, and it happens every time now, but not MSG or salt. No idea why.