The very little documentation about /proc/filesystems says it is a "list of supported file systems". I see a lot of file system modules in /lib/modules/linux_ver/kernel/fs, most of which do not appear in /proc/filesystems, nevertheless mount appears to have no problem using those file system modules.
So what is the use of /proc/filesystems? And why is it different from the list of filesystem modules?
If there is an available module for the file system you want to mount but it's not yet loaded and hence isn't yet shown in /proc/filesystems, then it will be loaded on-demand which it why you don't have any problem mounting.
After having mounted such a file system, then that file system type should have appeared in /proc/filesystems.
Hence it is a "list of supported file systems for which the driver is already loaded"; additional file systems may be added by loading the appropriate module.
