So i finally got an answer from geohot. He has no idea really why it wouldn't work on exynos device. Maybe by checking several modstring but 'ost probably by giving him an exynos device. I proposed to do some testing and also to give full access to my device through ssh but it looks like it doesn't help. He probably needs to attach to the kernel or something only doable locally.
Seeing the volatility of kernel exploits (doesn't survive a kernel update obviously..), the complexity of exploiting such a bug (dep, aslr etc..not something you invent in an hour..) and the fact that I don't care much about knox software warranty my a$$ ( have custom if I want to revert), ladies and gentlemen I will happily ignore knox and root my sm-t520
thanks for info