That's what the hypothesis u/tiffanyh EXPLICITLY says, and connects with the larger theory of Atlantis and Thule.

They already brought up the fairly discredited hypothesis that humans couldn't have built Stonehenge, ignoring the fact the prehistoric megaliths are actually fairly common, and "simple machines" are a fairly well known concept throughout much of history, and forced labor was VERY common throughout much of history.

No it isn't. The Sphinx water erosion idea says that the Sphinx is too old to have been built by the Egyptians, not that the Egyptians were incapable of building things like the Sphinx. The claimed evidence is apparent water erosion on the Sphinx and climate records, not the complexity of the Sphinx.

> They already brought up the fairly discredited hypothesis that humans couldn't have built Stonehenge

They didn't say humans couldn't build the Stonehenge, they claimed that nobody knows how it was done. They can clarify if they wish but I take this to be a rebuttal of your argument that "alternate" theories about ancient megaliths are obviously rooted in racism; Stonehenge is the subject of such interest despite being European. It's not motivated by some sort of racist desire to show that Britons are racially inferior people who can't stack some stones, it's just a legitimately interesting thing to wonder about.