@TinoPup is correct. Any giraffe that you see at a zoo in North America that is not a Masai giraffe is going to be a hybrid to some degree (there are
potentially a few purebred animals of the other two species knocking around but there are very,
very few of them) due to historical interbreeding before different species/subspecies were more formally recognized. This is a topic that comes on ZooChat quite regularly as we get new members because most zoos still label their animals as "reticulated" giraffe (or sometimes less frequently, "Rothschild's"), essentially to keep face to the public. All of the giraffe in North America that are not Masai giraffe are managed as a single population, where they are commonly referred to on ZooChat and in the field as "generic" giraffe, hence the program is known as the Generic Giraffe SSP. These hybrid animals are managed as ambassadors for their species in the wild. More zoos are wanting to switch over to supporting the purebred Masai giraffe population as animals are available; however, zoos still want to have giraffe, which is why the generic animals are still bred.
The information that was given to you on your behind-the-scenes tour is just simply not true. A lot of zoos employ people that are not directly involved in Animal Care or Animal Management to serve as tour guides and guest experience ambassadors, and more often than not they do not know the full story or all of the facts. It sounds to me as though your guide did not know how to answer your question and just decided to make something up. The San Diego Zoo has been managing Masai giraffe for many, many years, and no giraffe have ever been returned to Africa for any sort of in-situ management, and this is likely to never occur. The San Diego Zoo Safari Park manages a generic giraffe herd as well as a Masai giraffe herd in their two largest field habitats (East Africa and South Africa, respectively). For what it's worth, of the other zoos you listed, Louisville also manages Masai giraffe. The rest do all manage generic giraffe.