The source code is published under the MPL 2.0. You will find the link to the freely available source code if you read the game description. This link can also be found under More information > Links > Source code.
Viewing post in Sulphur Nimbus: Hel's Elixir comments
I sell the builds because money helps me to live. I've released the source code because I believe it's the right thing to do.
If you have the knowledge required to compile the game, then you can currently obtain a complete copy of the game for free. However, regarding the legality of redistribution: The source code is MPL, but the assets (e.g. the 'res' directory) are, at the time of writing, all rights reserved. This is in part because there are certain assets that I do not own and whose licenses are incompatible with the MPL. You have every right granted by the MPL for all forms of the code, but redistributing a copy and including the assets is, at best, on very shaky legal ground.
That said, I have no intention of fighting redistribution as long as it's not done in extremely bad faith, such as publishing a complete copy on a digital game storefront, or claiming that you made the game. Giving a copy to your friends is fine (and reusing any and all parts of my code is fine), but please encourage anyone who enjoys my work to consider buying the game to support me.
Code and assets being different licenses is not uncommon practice, and it is reflected in the itch.io Metadata. Commercial open source games also not unprecedented, although they are relatively rare. This article contains some examples that follow a similar model:
https://en.wikipedia.org/wiki/List_of_commercial_video_games_with_available_sour...