To continue with the coin flip analogy, consider a classical world where a coin flip is perfectly deterministic, given the initial conditions. In the usual way we think about determinism, it's still possible to test whether a coin is fair. Just flip it a bunch of times, a little differently each time, and see if you get a ratio close to 50/50. That's standard science.
In the superdeterministic view, you don't admit this kind of explanation. Instead you say that whatever outcome happened was determined by the initial conditions of the entire universe. If you get all heads, you can't say anything about the coin -- we merely conclude that your hands were predetermined to give it the kinds of initial conditions that would have led to that outcome.
Now imagine repeating an explanation like this for every experiment ever performed. It's on the level of answering "God did it" to every question. We don't do that in science because it kind of defeats the whole point.