From an initial assessment (Please correct us if wrong):
The seed of the randomness contains block time and player secrets.
Your secrets you know as you commit it.
The block time the miner knows pre-commit and is potentially guessable (as block times fall within a standard range).
As you know at least one sources of “randomness” (two if block times are not randomly distributed) it is most likely possible to calculate a bias. In addition to this, miners can further exploit the potential bias as they would know the block time going into the PRNG.
You don’t need to calculate bias from the sources directly as long as the sources of entropy don’t result in random distribution of generated numbers.
This implementation has 2 secrets that may not change, the only changed value number to number is block time.
Block times are generally not considered randomly distributed and also is known to the players after the game.
This implementation depends on the underlying random generator that determines the player secrets and over time how often a player reseeds their secret.
Lets consider that the players never change secrets, no matter what the secret is - because only block time is changed game to game the only source of entropy is ACTUALLY the block time as the other two are constants.