But by doing that he would need to maintain more code, which is unreasonable if it isn't something he wants.

And someone using the Notepad++ brand without his consent isn't cool, as if something goes wrong, people might assume that the original Notepad++ author is behind it, tarnishing his reputation.

If he doesn't want to make a macOS version that's on him, other people can fork it and make their own versions if they want, just make it obvious it's not from the original dev.