[decimon]

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.

In which dictionary may I find this?

[RegulatorCountry]

I'll have to look around for a source, but the meaning is clear in the original Latin.

“Millennium” is singular, ergo a singularity, and so the plural of that implies many singularities, a separation and discontinuity of eras.

“Millennia” is plural in and of itself, and implies continuity, an uninterrupted era.

So, “millennia” is the term the author should have chosen, and would have, if he had any grasp of Latin.