Based on that quote and how Duplicacy appears to use the file names themselves to identify blocks then it would make sense that it “natively” supports cross source de-duplication.
We know that at present Duplicati does NOT support that (though it has been mentioned before that it could) so perhaps a new Feature for “Cross source de-duplication” or “Cross-backup de-duplication” should be added with blank for Duplicati and ? for Duplicacy until somebody can verify our assumptions?