> They designed it to look like a differential backup model, but it's not.

The quote you pasted makes it sound like a reverse-differential incremental backup, like rdiff-backup implements (poorly). The most recent backup is a snapshot of the tree, restore points having changes are diffs applied cumulatively in reverse chronological order atop the current snapshot. You can simply toss out restore points starting from the tail older than the retention policy.

How is that not a differential backup model?