There are lots of things that were once commonly done by ancient peoples that we cannot do today because we lost the discrete knowledge that is separate from the primary knowledge of a task or subject. It's never the primary knowledge that gets lost and cannot be recreated. What does often get lost is the nuanced discrete knowledge that makes the primary knowledge applicable to the project. Let's take making vacuum tubes for example. Most of the world that once produced vacuum tubes by the millions has lost the discrete knowledge to do it again. Despite there being plenty of primary knowledge about how it should all work to make tubes, the discrete knowledge that came from those who were actually making them every day is nearly completely gone. The glass blowers who have developed and honed their techniques to become industrialized and mechanized are almost all extinct. No one new took their place so all that knowledge is lost. Without just that one parcel of discrete knowledge, there is virtually no way to make vacuum tubes at industrial scales again. Do you really think that the problem with recreating the moon missions is simply the lack of primary knowledge? Is a computer that is less powerful than a cheap home computer from the 80s is the problem here? No. The real problem is all the discrete knowledge about how to tweak the engineering and construction and operations of a vast array of equipment, processes and people are what is missing. It's the notes in the margin of the book that make the difference here. A fresh new copy of that book minus the margin notes is worthless because like a recipe, it is important to know the nuances rather than just the literal words. We lost the nuances and it makes the whole endeavor much harder to achieve again. So unless you know how to make Greek Fire the exact way the ancient Greeks used to, then you must at least entertain the idea that there's more to the story than just simply losing some technology.