This is kinda a science thing. It's not that you can falsify X (assuming X is true), it's that it would have been possible to prove it false.Not if it is true. The only way it could be falsified is if it were untrue. It you take the position that determinism is true you also have to take the position that it can't be falsified.
So, for example, we know that the Earth orbits the sun.
The way we could have falsified this statement is by sending probes to an appropriate vantage point, and having them watch the orbits of the sun and Earth. If the Earth actually didn't orbit the sun, this test would have falsified the statement.
I suppose it's about potential falsifiability.