US Courts have already ruled in the past that human authorship is required for copyright

Irrelevant to the issue at hand. Here, Silverman is the only one making a copyright claim. ChatGPT is not claiming a copyright on its output.

It'd be a logical conclusion as such that human authorship would also be required to justify a fair use defence.

I disagree. Nothing about "fair use" requires that the work be copyrighted on its own, or even copyrightable. It simply can't be subject to the original copyright.

A summary is a "transformative derivation". Even if that summary cannot be copyrighted on its for some reason, it is not subject to the copyright of the original work.