They were mass-produced and intentionally created, which is reason enough to be considered a B.O.W. as far a I know. A B.O.W. doesn't necessarily need to be marketable. I'm pretty sure there's more B.O.W.s that were not sold, although in Africa you can see the Ndesu, which is almost identical to the El Gigante, most lilely created the same way, as both had Plagas in them.
Someone correct me if I'm wrong though, I could be :O
You're thinking of B.O.W. in Umbrella's marketting department sense. A B.O.W. is just a weapon that was biologically-engineered. They can be based on people (eg. Umbrella's Tyrant line), and are wholely intentional in origin. From what we know, the El Gigante were the result of an enhanced Plaga parasite causing its host to grow along with it (just like the parasite in Del Lago), whereas the Verdugo were engineered to be Salazar's servants.