I think you possibly misinterpreted how crash games work. Crash game is not similar to dice where you have a server seed and client seed. Crash games have a chain of server seeds and they don't have client seeds, as the output has to be same for all the users who make a bet on a particular server seed. If they use client seeds the output of a particular seed will be different for different users. So instead of using client seeds, they use the hash of a future bitcoin block into the algorithm so that output is the same for all the users and provably fair. Other popular crash games like bustabit, stake etc, have also used the hash of a future bitcoin block instead of client seeds.