It seems that the regeneration process induced by the researchers is different than the natural regrowth pathway in Xenopus.

> At best, adult frogs normally grow back only a featureless, thin, cartilaginous spike, (…) Our procedure induced a regenerative response they normally never have, which resulted in bigger, more structured appendages. The bioreactor device triggered very complex downstream outcomes that bioengineers cannot yet micromanage directly.

(via https://www.sciencedaily.com/releases/2018/11/181106171813.h...)