The book's skepticism is partly from the test results from the August 26th, 2020 test of DF-21D and DF-26. So if we, as critical readers, want to confirm or refute these views, then it's a matter of determining whether those tests succeeded.
"When interviewed about it (sic. the August 26th tests) in November 2020,
... Months later, Vice Admiral Jeffery Trussler, the
deputy chief of naval operations for information warfare, implied that the US Navy was
."
So you can collate different sources, assess their credibility, then draw your conclusion. And viola, a way to assess the validity of these views. No one said that it would be trivial.