ErnestThornhill
It's perfectly possible to install GrapheneOS with OSes which are not listed in the prerequisites. The OSes listed in the prerequisites are the OSes on which the installers are tested. It's not reasonable for the development team to test the installers on every OS in existence.
I believe that's the reason for F0SSIL's question, but I'm afraid I don't have enough of a technical knowledge of the web installer or of that OS to know whether it can be made to work.