Ah, silly me! I totally forgot to check thematical sub-forum here (they are nearly useless most of the time for small games).
In this case the developers' behavior makes them scammers, plain and simple. At the Kickstarter they stated in the tiers description that:
A DRM-free digital copy of Moonlighter is yours to play! (PC, Mac, Linux)
yet, there is no DRM-free version for Linux at all
(I've checked, Humble Store also lacks it).
I hope they won't ditch Linux, but there are indeed a couple of things that point to an entirely separate pipeline for Linux, which probably leads to additional costs, delays and other issues.
Speaking of costs… I've been digging through CDProject's financial documents recently and found out GOG is in fact generates only losses for its parent company (not big ones, but still) as of the last year. :(
Something tells me GOG's descicion to cut expences on Linux is a vain attempt to rectify this.