The original game is a freeware Windows game by Locomalito, written in 2010. It was intended to have the look and feel of a ZX Spectrum game.
It was then rewritten and open sourced by MoonWatcher, and is under the CC-BYCC-BY licence. That means it can be re-used for any purpose, including commercial. Hence for example the commercial C64 version by Double Sided Games, and an actual ZX Spectrum version by RetroWorks.