I do not believe "this umbrella is wet" would work here, because the grammatical structure is different from "this is a wet umbrella".
In the first one, “umbrella” acts, by being “wet”.
In the second one, “this” is the one acting, by being “a wet umbrella”. Here, “wet” is merely an adnominal modifying “umbrella”.
It may sound like a silly difference, but in Hungarian, it makes all the difference when dealing with plurals. Adnominals do not agree with the noun, but adjectives do.
So, whereas on “Ezek a nedves esernyők”, “nedves” is singular, on “Ezek esernyők nedvesek”, it is forced to agree with “esernyők”.