Their so-called hybrids are a case of a little imagination and perhaps a lack of information allowing them to come to the wrong conclusion about what they are seeing. There are numerous species of wild rabbit in the U.S, and most areas have more than one species naturally occurring there. For example, we have Eastern Cottontails and Marsh Rabbits living around our place, and someone who knew about the one but not the other might think that the darker coat and shorter ears of the Marsh Rabbit was the result of some kind of cross-breeding with my Netherland Dwarfs. I have to admit, there have been times when I have seen a rabbit at a distance, and not been sure whether it's one of the natives, or maybe I didn't get a cage latched securely.
Also, though many domestic rabbits meet with a swift death when released, some do not; good habitat and a lack of predators can allow domestic releases to establish thriving populations, particularly in urban areas. This has become such a problem that some cities have even enacted legislation that requires that all pet rabbits be spayed or neutered, to avoid even accidental additions to their burgeoning feral populations.
If you want to use this buck, other than the fact that he lives in someone else's rabbitry and the possibility of their rabbits carrying something that your rabbits haven't already been exposed to, there's no reason not to. The "large Cottontail" that you mentioned in your first post was clearly a smallish, domestic rabbit, probably a crossbred, that happens to be Chestnut in color (Chestnut being the name of the
Agouti-based coloring like the wild rabbits). BTW, you said the buck in question is black in color. Black is recessive, as is the Rex coat; the buck got the gene for Black from both of his parents, but the gene for Rex only from his mother, so he's a normal-coated, Black domestic cross. Nothing exotic about him, nothing to be afraid of genetically.
