The developers emailed me this:
Hi there, luxebo!
Thank you for reaching out, I'm GM (removing name in case that's against policy), one of the game masters who've been assigned to assist the Survivors in Lumia Island. Apologies for the late response, we've been quite busy helping other Survivors across the world.
As I can see in your message, you're wondering about the developer's policy with regard to making a non-profit fan game based on Eternal Return.
We want to assist our players as much as we can, so they can continue their adventure as soon as possible!
To directly answer your query, you may create a non-profit fan game so long as you are guided by the policy that we have (Nimble Neuron IP Usage Policy)
Also, please know that hosting a game on any platform is not prohibited as long as it states that it's fan-made content and follows the rules and policy I mentioned.
I hope this clarifies your concern. And of course, if you need any help or if there's anything you're wondering about, just let me know.
Thank you and enjoy the rest of the week!
In addition, my response to the IP Usage Policy:
1) All content for ERW and EP derives from NN's game, ER.
3) The project(s) are entirely non-profit and designed as fun project(s) from myself.
4) Nothing here is made to damage or hurt anyone (or NN themselves).
5) I will take responsibility of everything involving third parties.