The interesting thing is though, what if evidence of the orions belt being conceptualised was stored in the Library of Alexandria and was lost in the fire? Would there be any other legitimate way of knowing that humans didn't have that knowledge back then?
I don't think the pyramids are that old, but it is an interesting theory.
Worth stressing that the Library of Alexandria
isn't that old. It was founded in the 3rd century BC (under the reign of Ptolmey Soter I, though
when within his reign isn't so clear), so to be a bit cliché, it was about as close to us as it was to the Pyramids of Giza. Most of its contents would likely have been closer to the time, or otherwise then contemporary recordings of ancient folklore; so even if there
was documentation of the origins of Leo and Orion's Belt - concepts established for hundreds, if not thousands of years by the time of the library, much younger than the Pyramids - they wouldn't necessarily have actually dated them unless there was a specific reference to an Egyptian monarch.
One shouldn't especially rely on the notion of 'Well, the might have been evidence, but it was destroyed', unless you have further evidence that such ever existed but cannot be found now.
The pyramids were not constructed by slaves. We have evidence that indicates they were compensated workers/farmers.
Pays to be a bit careful with that sort of statement. Both because the Pyramids (unless strictly referring to those at Giza) were built by differing kingdoms across a
long span of time, but also because the nature of slavery in this regard wouldn't necessarily have aligned with a modern perspective either. You could be in forced labour while still receiving a form of compensation, rather than say, a specific 'slave population' as we would imagine, and was imagined by the likes of Herodotus.
Edit: Also because people keep missing it, the supposed 'alignment' in this case is in terms of the relative positions and sizes of the Pyramids. Ie, they're supposedly set up in terms of location to look like how Orion's belt does.