Are these somehow different than Linux hard links, and is there really no equivalent?
Firstly, hard-links are a feature of the filesystem, MS Windows does not support hard-links on FAT32 devices. Nowadays this typically means USB memory sticks.
Secondly, NTFS supports three kinds of links and the exact capabilities and applicability may differ from Linux hard-links - for example, whether you can hard-link directories.
BackupPC is written in Perl. So it ought to be possible to find out what modules it depends on and whether those are availble on Windows. You can still backup from Windows clients. Presumably you want to backup to a Windows server?
The Requirements for BackupPc states that NTFS can't be used but doesn't explain why. (I don't know whether the MS SMB protocols allow for hard-link manipulation, or whether Linux NTFS drivers don't)
BackupPC uses hardlinks to pool files common to different backups. Therefore BackupPC's data store (TOPDIR) must point to a single file system that supports hardlinks. [...]
Any standard linux or unix file system supports hardlinks. NFS mounted file systems work too (provided the underlying file system supports hardlinks). But windows based FAT and NTFS file systems will not work.
You could email the author?
NTFS does indeed support links. However, very few people, even Microsoft-qualified people, seem to be aware of it, and very little Windows software has any facility for taking advantage of it. This article has more information.