The snapshot was deleted. You cannot restore from that snapshot anymore. It may not have deleted all the files in use by that snapshot, but it doesn't claim to do so.
Note, snapshots are not incremental in the simplest sense of just writing deltas from the previous backup, they follow the underlying lucene segments, which means merge events on the underlying indices will be reflected in the snapshots.
And my answer was that we don't have docs for that, and your best option is to look at the code for the existing plugins, and use that to guide you.