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.