Interestingly, this is not true. On the surface, one might think that ingenious is somehow based on the word genius. 大约3分07秒 在这里工作的员工也被 jobs 称为 「genius」(如字幕)。 但是如同高赞所说,这些 genius 受到的是sop的训.
If you use the word to mean an intelligent person then the plural can. Genius as a concept (of brilliance, inventiveness, etc.) predates genius as a person (one gifted with genius). It is perfectly correct and grammatical.
My answer always included the qualification that genii is limited to a particular meaning of genius. And one of the early.