I'm still not sure I fully understand what's going on at the low-level, but there is a "How it works" section on the debcow github page that at least made an attempt to penetrate my skull.

Edit: I guess the main thing it's doing is skipping the .tar archive extraction, and ref-linking the raw bytes from the .tar into new files? Extacting the .tar normally will create standard files, and those files would be reflink copied to the new location, but that still requires a more or less "normal file copy" during the .tar extraction. This really has greater implications for allowing generic reflinking from .tar archives, instead of just being limited to package installation. Could be interesting if it was handled automatically during .tar extraction. Or I could be misunderstanding, which is equally likely.