Consider the following two sentences:
For now, we cannot be sure how the machine worked back then. But once it existed, people would immediately have used it.
Sentences like these, where the first is "regular" and the second uses "would have", are used again and again over several paragraphs to lay out the history of the machine and what the author thinks is true about that history. From context, it seems clear that the author thinks the machine really did exist, and that people really did use it immediately once it existed.
Why does the author say "people would immediately have used it" instead of just "people immediately used it"? Is this an idiomatic way of allowing for the possibility that he might be wrong, or is it something else? (Non-native speaker here.)