Interesting find - thanks.
You don't really need to use the app; you can just use that website (particularly on a tablet rather than a little phone screen) - I just tried it by searching for a campground near my home, on this Android tablet.
I'm not sure about the WiFi comment: it needs a network connection, but I assume that could be WiFi or mobile.
One caution: like any database, it can be incomplete or out of date. That nearest camgroind to my home went private and no longer offers camping to the public, but it is still listed.