It's also relatively common for places to equate "last in North America" with "last publicly displayed in America". While a lot of Texas ranches do allow visitors, they aren't public zoos in the way a Philadelphia is, and are often way less transparent about their collection and what's on display, plus the less reputable of places you get, the more you start seeing all kinds of genetically impure animals- I'd be curious to see proof of places in the private sector having purebred Dama Gazelle, and that they aren't a generic mix. I could be wrong, but I'd suspect a lot are actually the latter.