If I get it right, there is no such thing as deltas. Every generation seems to virtually stand for its own. However, double chunks of data (no matter if being caused by different machines or generations or whatever) are deduplicated in a generalized way. Nice design! As check for duplicate data made before transmission, this could be also very efficient (however, one could still transmit with rsync and than use obnam).
I'll stop spamming this list now with obnam features, however the delta question is the one that arises for sure when other backup systems are mentioned here ;)