It does get data from the title key site, but it also fetches metadata for game info, boxart and such. This could have been fetched directly from Nintendo and cached, it would be slower but at least it would always be up to date. It's the way I would have designed it, no point in relying on a central server when it's not necessary.
Now that Wii U USB Helper is officially abandoned, it's using a 3rd party server for the metadata, which means the guy who made the launcher is the one that has to update it. I would hope he has a script that does it daily or so, but he may have it set to once a week or every other week. Anyway, eventually when it's updated the game will show up.
The JSON file most likely contains the title keys, you can open it in a text editor to check.