As far as I know, でしかない means: no more than or merely, but in this sentence that meaning doesn't seem to fit.
So, what exactly does this expression mean in this sentence?
Japanese Language Stack Exchange is a question and answer site for students, teachers, and linguists wanting to discuss the finer points of the Japanese language. It only takes a minute to sign up.Sign up to join this community
Assuming it was written by a native speaker, I would call this a humorous sentence rather than a perfectly standard one. The sentence literally does mean:
Majoring in that means [someone] is merely a genius!
But it effectively means:
Majoring in that means [someone] is nothing but a genius!
しかない is normally used in a negative way ("only", "merely", "no more than", etc), but here it is used to refer to something desirable, which is why you were puzzled, right? In reality, people may say something like this half-jokingly.
Don't try them at home unless you're really fluent in Japanese.