Erm, that's because of the immortality granted by Edo Tensei. If he had taken that while he was alive, he wouldn't have been able to move or use Ninjutsu, since Rasenshuriken destroys your chakra network according to Tsunade. Even he can't get around that.
Kabuto said that there were no currently living ninja that were as strong as Hashirama.
No, he just said "There aren't ninjas on the same level of the first hokage Senju Hashirama anymore." He never said currently living.
....That's what that means. "Anymore", implying there once were some on his level (with Hiruzen known as being above him), but that there "no longer were", implying no one alive was, which is what he meant.
