I'm just supposing here and no doubt others will correct me, but . . . .
It seems to me that your reference is unclear as to what it means by "separate file systems." It could be interpreted to mean either separate partitions (systems of files) or separate file systems (systems for storing and retrieving files, such as ext3 and reiserfs).
RTFM is all well and good as long as the manual is clearly written.
No wonder you have questions.
The only hard links I have ever used are the ones inherent in the system. I've never tried to create my own. I have lots of symlinks.
But I did find this which may be some help, but I still found it kind of muddy: