No, they are library fields.
I was talking about experience with a new field called Compressoin that was added i think to 11.1. At the time if you upgraded to this version, that had this new field, it would not be filled for any of the existing library items. (which is the correct behaviour)
Only future imports would have it filled. The only way to populate the compression field for existing library items was to either update library from tags or a re-import, both of which force MC to read the file again.
So if they create a new field Filename(folder), my feeling is that it will not be filled when you upgrade to the version that has it, the only way would be to re-read the file again, and this would mean reading all the other tags in the file and replacing the content of library fields with their file tag counterpart.
If your file tags are not in sync with the library, (i keep field updates in the database only) then all field updates made to the library for the tags that are also present in that file will be lost !!..unless they do it in such a way that only the new field is populated and ignores the other fields, which sounds like more work and isn't the default way of doing it, which is read all tags.
This is why maybe an expression might be easier to implement and could even have more options with it
They could of course do it both ways, offer a folder expression as well as a folder field, i would just ignore the field, and use the expression in this case.