Some peonies can grow from bits of roots left behind, it's called adventitious rooting. I didn't know Itohs could be propagated that way, although there was some documentation on an experiment with bits of Itoh stem with very little root attached on Southern Peony Blog. This should take you right to her last post on the experiment with a link to the start, if not, the post was from May 2013.

I have several peonies that reproduce from bits left behind after dividing/moving but they're all normal herbaceous. Lovely Rose has produced lots, both Circus Circus and Raspberry Rumba seem to do the same.

Congratulations on your new Bartzella! Isn't it nice to get an unexpected surprise of that kind?