It's inconsistent, really. The
Ask a CDev portions state that Trolls can regrow lost fingers and toes, but not major limbs like an entire leg or arm, whereas the
Chronicle series only states that Trolls can regrow limbs (which fingers and toes technically are) without extrapolating overly. In the short story
The Judgment it's specifically stated that Trolls can't regrow major limbs, but can regrow up to a hand or foot. In
Shadows of the Horde the Zandalari can almost instantly repair arrow punctures, recover from a collapsed lung, as well as reset and almost instantly use a broken limb.
As for Zul'jin, could be either one or could be that he was simply too injured to regenerate. Alternatively, his wounds were made with magical weaponry to prevent regeneration, like with Trol'kalar.
- - - Updated - - -
Unsure if it's really Life specifically - could be Spirit or even Death. When Bwonsamdi took away Vol'jin's "sense of being a Troll," which included his ability to regenerate from wounds, the actual substance of "being a Troll" was described as a sphere of shadowy, wispy energy before it was restored to Vol'jin and he began rapidly recovering from his injuries during the events of
Shadows of the Horde. That doesn't really sound like Life magic, but it could also be that the substance of it varies from patron Loa to patron Loa (and Death would make sense for Bwonsamdi, given his portfolio).