Earlier quoted context omitted.
Millenniums is less common, but still valid. https://www.merriam-webster.com/dictionary/millennium >> "plural millennia\ mə- ˈle- nē- ə \ or millenniums" Dictionaries are based on real-world usage, so a lexicographer found enough usage to print it.
The relevant dictionaries for an article in the UK Independent would be the OED, Collins, or Cambridge. https://www.lexico.com/definition/millennium https://www.collinsdictionary.com/dictionary/english/millenn... https://dictionary.cambridge.org/dictionary/english/millenni...
I couldn't find freely-available online versions of those when I looked. They all say milleniums is valid.