Monosodium glutamate is a salt not an amino acid. It does not disassociate perfectly (or instantly) in the gut and uptake through the digestive system is slow [1].
There are several steps before your metabolic system can turn MSG into amino acids and surprise, surprise: until then it behaves like a different chemical.
[1] https://pubchem.ncbi.nlm.nih.gov/compound/Monosodium-glutama...
The first sentence in Section 10.2 (Absorption, Distribution and Excretion): "Glutamate is absorbed from the gut by an active transport system specific for amino acids."
I'm not convinced you read what you just cited.
I say that to say you make a lot of them. And you eat a lot of them whenever you eat organic matter.
And to your point, even water can be deadly if you drink too much too fast. That doesn't mean you have a water allergy.