An over-simplification, but an evocative one:
- The social sciences are contentious, their predictions questionable.
- And yet social sciences use the scientific method; AI predictions generally don't.
- Hence predictions involving human-level AI should be treated as less certain than any prediction in the social sciences.
To summarise the argument further.
"A lot of people talk rubbish about AI. Therefore most existing predictions are not very certain."
That doesn't in itself mean that it's hard to predict AI - merely that there are many existing predictions which aren't that good. Whether we could do better if we (to take the given example) used the scientific method isn't something the argument covers.
I don't really see how we could do that. Yes, most predictions are rubbish - but a lot are rubbish because predicting AI is not something we have good ways of doing.