I think it's a mistake to assume no Githyanki trade ever.
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and I'm saying there's no evidence for that.

You're making my point. Which suggests to me that you might not actually even understand my point.
The only evidence we have is that the Githyanki don't care about the armor. Instead of assuming that's a mistake, we can instead consider reasons why that might be.
One possibility is that the Githyanki trade the armor. This is within the realm of reason. We have no evidence for it, but it does explain why the Githyanki don't comment on the armor. Because they don't care. Because plenty of people wear the armor.
Another possibility is that the Military Caste of the Githyanki don't respect the lesser Caste that makes the armor. Maybe they think: if someone got a hold of this armor, obviously the smith who made it did an inferior job because it didn't protect the Githyanki who originally wore it. Thus, why reclaim it?
Another possibility is that the military branch of the Githyanki think there's so much of that armor floating around that they don't care.
Yet another possibility is that the Githyanki are simply alien in thought and can't imagine why you would care what someone else is wearing.
The point is, there are possibilities. The idea that the Githyanki would comment on the armor or attack someone over the armor is not a foregone conclusion. It's nothing more than the first obvious thought that comes to anyone's mind.
If Larian decides to go that direction, I'll accept it. I just don't think it's a particularly good idea. I think there are better, more interesting possible explanations worth considering.