Places that are searchable from the uploader or from the Location box on Explore aren’t necessarily stored in the iNat database, they’re pulled in real time from Google’s geolocating service. In this case, iNat doesn’t have a place for that park, so if you want one, you’ll have to import it.
One option is to take the outline from OSM (https://www.openstreetmap.org/relation/3740412) and pull it in with overpass turbo.