Is it true that the best prop firms these days only hire math savants as traders?

Having gotten offers from most of the top prop firms for internships and/or full time, I can say that no, if you truly mean savant, but I do have to say that the on-site interview at Five Rings really separated the boys from the men when it came to mental agility.

Being able to think quickly is very important. Conceptually, the brainteasers are not very hard, most firms ask high school level questions. However if you are the type of person who spends hours on a problem set, the format of the interview will be very difficult.

If you did competitive math in high school and was very good at it, I would say you would be well prepared for the brainteasers.

The difficulty of these interviews at all but a handful of firms are severely overstated.

If you can't go out of your way to get a working understanding of Bayes' Rule then you are probably too stupid or lazy to work at any of these places anyway.