But the US authorities are quite clear that a work that is purely AI generated can never qualify for copyright protection.

Which law says this? The government is certainly discussing the problem, but I wasn't aware of any legislation.

If there is such a law, it seems to overlook an important point: an algorithm - an AI - is itself an expression of human intelligence. Having a computer carry out an algorithm for summarizing content can be indistinguishable from a person having a pattern they follow for writing summaries.