If they show the hashed seed before you bet, and the process using the server seed (provided afterward) and your client seed results in the output along with their server seed hashing into the hashed seed then it's perfectly verifiable.
Suppose my server seed is X and the result of it and your client seed would be tails (in a head/tails game).
I then hash the seed with SHA-256 and show it to you publicly. After you bet I reveal the server seed and boom: you can verify everything since I can't lie about the hash.
Please correct me if I have made an error in any way.
You are correct, but my point is that: is the "hashed seed" they show you, really the seed used to run the game?
for example they can just generate a random hash seed (asdbfj891y23basd) and display it on their website but then actually, their game will use a different hash seed (138sdf71982789zcxz). then when you lose, they will of course show you the server seed which solves your seed. but in reality it is totally different.