A post-mortem also has the benefit of hindsight. Decisions that were clear and definite at the time might not make much sense when viewed with data obtained later in the project's lifespan. There's also an issue with "selective memory" where something might not be remembered with the same clarity, or may be conflated with other events. Confident statements like "We knew this one thing wouldn't have worked" might actually have been "We weren't sure if this would work so we tried several things. They all didn't work.". Consider anyone writing about their past as an unreliable narrator. True, they may be the best and most knowledgeable narrator we have, but they do not have an objective perspective on whatever they were creating. They have their own biases and reasons for the stories they present in a post-mortem. Treat a post-mortem like you would treat an auto-biography of a famous person: a primary source with an agenda to show the subject in the best way possible.
A post-mortem also has the benefit of hindsight. Decisions that were clear and definite at the time might not make much sense when viewed with data obtained later in the project's lifespan. There's also an issue with "selective memory" where something might not be remembered with the same clarity, or may be conflated with other events. Confident statements like "We knew this one thing wouldn't have worked" might actually have been "We weren't sure if this would work so we tried several things. They all didn't work.". Consider anyone writing about their past as an unreliable narrator. True, they may be the best and most knowledgeable narrator we have, but they do not have an objective perspective on whatever they were creating. They have their own biases and reasons for the stories they present in a post-mortem. Treat a post-mortem like you would treat an auto-biography of a famous person: a primary source with an agenda to show the subject in the best way possible.