A Shinigami visiting Zoro is way out there, even for One Piece, that I'm inclined to think there's some catch to it. Perhaps a hallucination because he is approaching death. If so, the mink medicine after effects aren't being used as a ticking time variable, like I thought, but an actual ordeal Zoro must overcome. The Grim Reaper is but a visualization of Zoro's struggle.
Or, he is becoming in tune with Enma following his fight with King, and this is the process in which he will achieve a black blade. Oda never gave us a personified Enma during his SBS, even though he probably received requests for it. And the idea of swords possessing sentience was hinted way back in LogueTown, and further implied by Kozaburo.
This Shinigami (presumably Enma) appearing before Zoro, Bleach style, is the idea being taken to an extreme extent and conclusion.