The Pyramids have a clear design evolution. We know the steps they made that led up to Giza. We know the hurdles they had to overcome and that they figured them out through experimentation.
When we refer to ancient advancements. Generally, when we say "we don't know how they made them" we mean literally just that. We don't know how they made them. We can come up with theories using the technology of the time that would have been effective (generally involving amounts of manpower that would make a modern person shudder at the excess involved), but I don't think most scientist believe that it would have been impossible to build with their technology, just that we can't prove the methodology.
When we refer to ancient advancements. Generally, when we say "we don't know how they made them" we mean literally just that. We don't know how they made them. We can come up with theories using the technology of the time that would have been effective (generally involving amounts of manpower that would make a modern person shudder at the excess involved), but I don't think most scientist believe that it would have been impossible to build with their technology, just that we can't prove the methodology.