I have installed many rads in my time on Planet Earth. None ever came "wet". Not trying to start a fight, now. Anybody else ever get one delivered wet?
Me thinks the key word was used
Antifreeze has stuff in it that is sacrificed to combat the nasties that occur while in use. If simply drained & allowed to dry, the nasties can continue to corrode the metal.
When removing a used radiator for storage, I was taught to wash it inside & out.
The original post was talking about 'claimed' new old stock, so the potential buyer should examine/ test the radiator before purchase.
Definitions can be different for different people. . . .
Some will call a part 'new' when they sell it because they only used it for 3 months .
A used car salesman may use words like 'mild oxidation' to describe what I'd call rust holes . . . .