Genius as a concept (of brilliance, inventiveness, etc.) predates genius as a person (one gifted with genius). My answer always included the qualification that genii is limited to a particular meaning of genius. If you use the word to mean an intelligent person then the plural can.
Interestingly, this is not true. Genius is the correct spelling that comes directly from latin: On the surface, one might think that ingenious is somehow based on the word genius.
It is perfectly correct and grammatical. And one of the early.