[fragmede]

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

[VorpalWay]

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.

[microtonal]

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

    $ touch "foo:bar"

In the Terminal, then Finder renders it as foo/bar.

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

[kps]

APFS stores it as "foo:bar". (I don't think the filesystem itself cares what's in a name.)