Can anyone answer if "could not have built" in the paragraph is a speculation or past incapability?
"......Believers in alien visits to our planet argue that there is historical evidence to support their view. They point to the pyramids in Egypt and Mexico and the size of the Easter Island statues, saying that people in ancient times could not have built them........"
Hypothetical, counterfactual inability. In fact they were able to build them, thanks to the alien visits.
What is the difference between "couldn't build them" = were unable to build them (and so did not build them) and "could not have built them" = would have been unable to build them (in other circumstances, but did build them).5
Student or Learner