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........"
The below is an extract from a grammar website. If "could have pp" is a speculation, there should be a negative speculation as well, but the website seems to say negative one is always an incapability, so I'm confused. If so, what is the difference between "couldn't build them" and "could not have built them"? Your kind answer will be highly appreciated.
=======================(from a grammar website)
1.We can use 'couldn't have' to talk about something we were not capable of doing.
· I couldn't have managed without you.
* I couldn't have got the job. He was always going to appoint his nephew
2. We can use 'could have' to speculate about what has happened. (We can also use 'may have' or 'might have' in these situations.)
- She could have taken the earlier train.
- Simon could have told her.