The laws of physics are symmetrical, and if the future can be known perfectly given the past, so too, the past can be perfectly known given the future.

If you're going to ignore the causal structure and step outside time and look over the Block Universe, then you might as well say that the past was already determined 50 years later.

You might as well say that you can't possibly choose to run into the burning orphanage, because your decision was fully determined by the future fact that the child was saved.

If you are going to talk about time, the future comes after the present.

If you are going to talk about causal structure, the present screens off the past.

If you are going to throw that away and talk about determinism, then you're talking about a timeless mathematical object and it makes no sense to use words like "before".