Current macOS Finder let's you name files with a slash in them, rendered as a : in the terminal.

Isn't that just what the original article that we all commented on described? I don't understand what you are trying to add to the conversation here.

I think that the slash might be the rendering though? If you

    $ touch "foo:bar"

So who is lying, how is it stored in the directory entry in APFS itself?