The discussion around the side effects around the drugs just before the appearance of the grim reaper suggest it'd probably just turn out to be an illusion caused by the side effect of the drug pushing Zoro to the brink of death and attempting to fight death to survive
And if you look at the panels closely, you'd see that Oda drew an aerial panel without details and in the closer panels, the background is twirly
It's like the Oden drawing in front of the scabbard that turned out to be Kanjuro's doing. Oda likes these kind of stuffs. Doubt it'd actually turn out to be a real grim reaper