My throat burns when I eat more than just a dash. I wasn't diagnosed with an allergy though. So YMMV.
Just because the human body produces small quantities of it at a time doesn't mean that you can flood the digestive tract with orders of magnitude more and expect no ill effect.
An amino acid is a building block for proteins. It isn't something you produce small quantities of, it's a basic component of how organic life is made.
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.
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.
Glutamate is an amino aid and a neurotransmitter, your body produces it. Per [1], "It is used by every major excitatory function in the vertebrate brain, accounting in total for well over 90% of the synaptic connections in the human brain." Per [2], "Glutamate itself is ubiquitous in biological life."
I don't legitimately see how someone can have an allergy to it. It seems to be a basic building block of biological life. It's like saying you are allergic to DNA.
[0] https://en.wikipedia.org/wiki/Glutamic_acid [1] https://en.wikipedia.org/wiki/Glutamate_(neurotransmitter) [2] https://en.wikipedia.org/wiki/Glutamate_flavoring#Natural_oc...