[decimon]

His having written “millenniums” doesn’t help, either.

Just checked a dictionary and the author is correct.

[RegulatorCountry]

He is referring to millennia, in series, one flowing uninterrupted into the next.

“Millenniums” would be differing and separated eras of 1,000 years.

The two are not always interchangeable. The author misuses the term in this instance.