Do adult mammalian cochlear inner hair cells regenerate?
Short answer
The current consensus is that hair cells in the cochlea of humans do not regenerate spontaneously.
Background
I took the liberty to show the linked paper to a colleague of mine. This guy has been doing histology on the inner ear for his entire professional career.
He pointed out that the consensus is that in mammals, cochlear hair cells do not regenerate. This as opposed to birds and fish where they can regenerate. In mammals, vestibular hair cells can also regenerate (Santaolalla et al., 2013).
Where do these 'supernumerary inner hair cells' (sIHCs) the authors of the paper show in their (quite stunning) EM photos come from? I wish to point you to the last lines of the Discussion of your linked paper (Rask-Anderson et al., 2017):

Taken together, it cannot be settled if the sIHC represent renewed or redundant accessory IHCs. Further molecular studies are needed to verify if the regenerative capacity of the human auditory periphery might have been underestimated.

In other words, the extra hair cells may have appeared during development in utero, or perhaps due to regenerative processes later on. Note that the inner ear in mammals is fully developed and doesn't grow anymore in size. The EM photos in the article don't allow for the identification of functionality either. Also note that sIHCs have been identified in the 1800s in other animals and that the beauty of this paper lies more in the fact that it shows that sIHCs exist in humans, rather than showing that IHCs regenerate in man (that would likely not appear in the Upsula journal of medical sciences, but in Nature or the likes). On a side note - the authors of the paper are giants in the field.
Literature
- Rask-Andersen, Ups J Med Sci (2017); 122(1): 1–19
- Santaolalla et al., Neural Regen Res (2013); 8(24): 2284–9