BRAFF ZACKLIN said:
zeref meanwhile solved concepts far above what anyone in real life or in FT was ever even close to achieving
Which is the entirety of legitimate reasoning for him being a genius in the subject of magic.
Just that if you have magic the gap between cellphone and time travel or creating life is possibly a lot smaller than in real life.
The fact that he is a lot smarter than everyone else in the verse stays non the less.
So you still can not compare it to real life world scientists based on the the things he invented. Neither can you assume the gap between their magical technologies is the same as the developement gap that those technologies would have when accomplished by scientific means.
You only can judge him based on the gap between him and the other magicians, which is large and can be used to argue a ranking as genius in magic.
I can also give you a more accurate example on such a case from another fiction: Any magician in To Aru Majutsu No Index, can easily create a device that lets you levitate. On the other hand science would need technology on the stand of 30 years further than the normal world to do the same.
So why can magicians do it in the world with current technology, but a scientist only after 30 years?
Because a magical device can directly manipulate gravity, while a scientific device needs to find a way to influence gravitons by scientific means first.
So for a magic society invention of levitation devices came before cellphones (like 2000 years), while for a scientific society it came 30 years after.
That doesn't make the magician that invented flight more intelligent than the scientist though. With the methods he had avaiable the gap between inventing a phone and levitation is just smaller than for a scientist using real life science only.