So if you have the schema and the DBMS, you probably know how data is arranged in the files ("files" in the filesystem sense).
The parent asks "how is it not a file layout" not "can you guess the file layout?" given it.
I am a human, you know I have a kidney, but I am not a kidney.
If you send a copy of the code, is that sending the code? If it is, what about sending a copy of the code with a Caesar Shift?
Is your argument that government agencies should also withhold the names of filing cabinet manufacturers? :)
Just that it's a file layout. Or even if you strictly define a file layout as say an ext4, NTFS, or FAT file tree, that revealing the schema is revealing the file layout.
I don't know why they don't want to reveal file layouts, but for whatever reason, they decided it was "per se" exempt regardless of the security implications.
It's obviously not a file format. The same SQL schema can generate N different files, with N different layouts, for N different databases. By the logic you're using ("schema" + "database vendor" = "file format"), a Word document outline is also a file format.