If I recall correctly, there was also the issue that a Nintendo 64 ROM of their game would be fundamentally incompatible with Steam, which (as many forget) is technically their DRM solution. I could be wrong, of course.
You are free to publish any ROM to any system, it's a basic right against both monopolies and freedom of speech restrictions. What you can't do is to ilegally pull propietary dependencies without permission.
The problem I'm pointing out is that it's a work based on a Valve property that fundamentally cannot be tied to the DRM because it's "just" a ROM.
I believe this came up when the creator was talking about libdragon-- Valve has been more forgiving of other games like Hunt Down the Freeman and whatnot because they're native executables with the Steam DRM, which video games based on Valve properties necessarily must have. Portal 64 simply cannot do this, because Steam is not a Nintendo 64 application.