You can write that blog post because you are a human, and your summary qualifies for copyright protection, because it is the unique output of a human based on reading the copywrited material.
But the US authorities are quite clear that a work that is purely AI generated can never qualify for copyright protection. Yet since it is based on the synthesis of works under copyright, it can't really be considered public domain either. Otherwise you could ask the AI "Write me a summary of this book that has exactly the same number of words", and likely get a direct copy of the book which is clear of copyright.
I think that these AI companies are going to face a reckoning, when it is ruled that they misappropriated all this content that they didn't explicitly license for use, and all their output is just fringing by definition.