Technically the game itself was partly funded through kickstarter.
The other half of the reward was early access to the game.
There was an option to not get the console and to just get early access to the game.
Either way I agree with @Vampirics that “the right to use Arduventure’s graphics” was not really included or implied at any stage, so it’s up to Team ARG (or perhaps the artist specifically) as to what licence the graphics are released under.
I agree that they probably should have a licence for the graphics though.
The line between code and graphics gets blurred when the graphics are converted into arrays within the code itself so it’s understandable that many people don’t think about it.
A lot of people making games don’t really know about all the intricacies of licensing (I admit that I myself have neglected to provide a licence for the graphics included in my games), and we really ought to have a guide somewhere explaining it, but we don’t.