Thank you for your comment and interest. Regarding the paper, I also mentioned that, "Since both peripheral and central neuronal injury may be permanent and regeneration is extremely rare and slow,76−78 brainstem injury may be long-lasting as well."

76-78 are references to studies showing so. Peripheral nerves appear to have stronger regenerative capacity compared to brain nerves. But it doesn't mean that brain (or peripheral) nerve regeneration isn't impossible. It can happen, but studies show it's rare and slow.