That being said: I never imagined Saru to be coming from a totally pre-technology world. I always thought of him coming from, like, a post-apocalyptic Hunger Game's world.
Who knows, maybe that's what Kaminar actually is. Ever heard of Yor, The Hunter From The Future?

Because I find it hard to believe that he can become second officer to Georgiou, when she was already a Leutnant, and he didn't even new basic math.
Well, Saru would have to go to the Academy first - it's not like they immediately made him an officer. And where did you get the idea he doesn't know math? He's obviously intelligent, he deciphered the instructions on the Ba'ul communications device.