I wonder if they use a rolling checksum too, to avoid duplicating a complete file if only a view bytes shifted (for example adding a line of text in the beginning of a file)
It probably wouldn't hit the most important cases either, dedup is typically most powerful & valuable on large media files, software packages, disk ISO's, and the like which do not frequently have arbitrary text inserted at the start of the file!
The backup tool bup (https://github.com/bup/bup) does this.