“The past has a probability of 100%” is a statement that helps avoid getting too into this argument (for better or worse). It can mean “the past has happened, and probability no longer has any effect on it”, which is where you seem to be coming from. We shouldn’t base too many of our decisions on the theoretical chance of something specific happening if the chance of it happening is already in the past (sunk cost fallacy, as an example).
But probability is fun! And even if we can’t change the past, looking at the theoretical probabilities can give us appreciation for how special the present is by examining how special the past was. So when somebody talks about a past event with a probability, they are talking about theoretical events that didn’t happen, not the 100% of the past. They are talking about how special the current state of things are, and how randomness can measure the theoretical chances.
It’s misused fairly often (so what if my combination of genes has a small chance of existing, I’m me and even if I was different I’d just be a different me that also had a small chance of existing), but generally thinking of “past random chance” as “how special past events are” instead will probably help you understand what people are talking about.
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