Note that they said "Most involved" Russia, for instance, has always been the modern "Sick man of Europe" since the fall of the USSR. It's imperial aspirations don't extend as far. And it's relationship to the historic Core of the US and Western Europe, is as a semi-peripheral nation trying to coalesce a regional sphere of influence with itself as the center of gravity. None of that makes it a Core country though.

Maybe if the current world system collapses, and it filled that vacuum. But that hasn't happened.

Imperial Core refers to the World Systems Theory of International Relations, first put forward by Immanuel Wallerstien. I would suggest you read up on the topic before making half-baked responses like this.