It's not complicated. It's been studied directly. It is an easy study to conduct. Give people MSG. Do they become symptomatic? The answer is no. This is the result you would expect, given the simple molecular structure of MSG and the fundamental role glutamic acid plays in the human body.
If we had learned about table salt in the 1910s and knew it primarily by its formula name NaCl, people would believe all manner of weird things about salt, too. In fact, people do believe weird things about salt; there is a widespread and unsupported belief that moderation of sodium intake is a key public health concern, which it is not.
It's the same with MSG. Dissolve MSG and you get sodium and an amino acid that is so important to our bodies that we produce it ourselves, without sourcing it from our food or the environment; an amino acid that is thus, unsurprisingly, hugely prevalent in foods we "naturally" eat.