One true way to be sure that seed phrase is truly random is by using properly tested casino dices, so you don't have to trust any chips or computers that create fake random results that can be reproduced.
I'm not paranoid enough to do that, I just wanted to know if this kind of function is verifiable. It doesn't matter if I roll a dice or not when the overwhelming majority uses hardware and software wallets. If, say, Trezor is proved to generate predictable entropy and steals everyone's funds in just one moment, it'll harm the entire bitcoin economy.