The previous posts are to a twitter thread that's been taken down, and the preview of a post that requires a $1000 subscription. This post however is freely available (for now at least).
A summary isn't allowed under US copyright law. The copyright office calls them "condensations", and they are considered derivative works.
His use was likely not within US copyright law. "Effect of the use upon the potential market for or value of the copyrighted work" is one of four factors a judge should use to decide if fair use applies, and it is clear that publishing the main information from an article, information which is not available elsewhere, freely, severely degrades the market for the original.
