The news article, as well as the published work, state that:
Cartilage regeneration appears to occur through gene expression changes in pre-existing chondrocytes, rather than stem or progenitor cell proliferation.
Long-term side effects will need to researched of course and there might still be stem cells involved in the process, however that doesn't seem to be the case according to what we know as of now.
Gods I fucking hate that app (and the way Reddit's mobile site tries to shove it down your throat). Welcome!