I was joking about there being five. Starduino is the only one I could think of.
As you’ve evidently realised, Circuit Dude does have source code available, but it’s not under an open source licence, much like Midnight Wild. (Though I’m not sure if the source code for Circuit Dude is up to date or not, it might only contain the older levels.)
There’s a few games that don’t have any licences, but all the examples I’m aware of have source code available.
So Starduino is genuinely the only truly closed source Arduboy game that I can think of.
Everything else is at least source available as far as I’m aware.