Let P be a complete description of the ancient past and let L be the laws. Lewis agrees that our freedom includes the ability to act in a way that falsifies the conjunction P&L but denies that it includes the ability to act in such a way that were we to act so, P would be false.
But here is a plausible thesis. Fundamental particles are essentially tied to laws of nature. There would be no electrons or photons if the laws of electrodynamics were different. This is clear on the Aristotelian picture on which laws are grounded in the powers of objects, but is also plausible without that picture.
Given this plausible thesis, P entails L. And hence P&L is logically equivalent to P. Thus if we can act in a way that falsifies the conjunction P&L, we can act in a way that falsifies P. Lewis denies the thesis, but it is still plausible.