I simply said that the filesystem is unspecified. Whether it's a real one or not doesn't matter to me. The reason why I would like to make it special and hide it from the OS is to reduce the complexity of the program and improve its reliability especially when facing future changes.
An approximation of this design is separate /boot, but IMHO the fundamentally bad idea which opened the whole can of worms is that grub reads ANY mountable filesystem, even if the filesystem was as simple as VFAT. But I understand defending this version of the argument is very difficult, and anyway all the effort has already been expended in getting every filesystem's every version work as well as possible. We might just as well roll with it, but in my opinion there is a lesson here, which says "We should not expect boatloaders to be as good as our operating systems when dealing with filesystems and hardware". Or something along the lines.
After all, people seriously thought about using a Linux kernel just to load another Linux kernel. I'm sure there's a yo dawg joke here somewhere.