Wikipedia article and OSM tag definitions don't exactly align. I would also call this an open-air museum in some kind of way but I wouldn't tag all open-air museums as theme parks. Having been here several times, I've been more under the impression that this is some sort of historic park rather than something like a theme park. Even if this isn't exactly a park under every OSM definition, it's maintained and open to the public like one. leisure=park is a very broad and neutral tag as opposed to tourism=theme_park.