I don't know the math of quantum mechanics. My layman's understanding includes the belief that quanum state evolution is deterministic (described by the Shrodinger equation). I may well be wrong about this.

Either way, my point was that before Maxwell's equations for electromagnetism, and the understanding that light was a form of EM, science didn't have anything like a complete description of physics. So it was hard to say whether physics was deterministic, even though the existing Newtonian law of gravity was. Once there was an attempt at a Law of Everything, even though it was refined over time, there was at least strong evidence for determinism.