It is physics based.
A developer of the dice stated in this forum thread:
Hey all, one of the devs on the dice here.
The dice are all physics based and not an animation. With it being physics, a lot of factors get involved. With how we throw the dice, what dice are thrown, how hard, and even the screen size will change how the dice collide with each other to give the final result.
First we randomize the start rotation so even if we dropped the dice without throwing them it would be random.
Second we also throw the dice in a random direction with a random force, this makes it even more random!
Third we also add an additional throwing rotational spin so when things collide they will collide differently even if for some reason the force and start rotation happened to be the same which that makes it even more random!!
So while a randomizer seed can be the same for 2 people other factors that happen on your computer can and will change how the physics run and give you a different result.
If we ever see things with how the randomization works that cause an issue we will absolutely fix it to make sure its as 'random' as it can be. If we have to we will absolutely look into other things like https://blog.cloudflare.com/randomness-101-lavarand-in-production/ ... well maybe not that extreme but we will look into other ways if it ever becomes an issue.