See it's close to The Time Machine's main plot in the sense that it follows concepts of fantastical nature over traditional sci fi. There are many categories, but science fantasy is definitely the one in here.
Sci-fi can cover a wide spectrum. Not many people consider say the Frankenstein novel sci-fi, but it is.
I thought it was pretty well accepted that Frankenstein is sci-fi? In all the classes I studied it in, my professors called it the first sci-fi novel (even in a class that dealt with romantic literature).
In any case, I guess it is a bit hard to classify it. Maybe science fantasy is the best way to describe it.