I think the author of the post was referring to the fact that Belgium is a multinational state, comprised of Dutch-speaking Flanders and French-speaking Wallonia.

But it isn't, here. The state of Belgium created itself by secession from the United Kingdom of the Netherlands, and its populace generally comprises two nations, Flanders (Flemish) and Wallonia (French), neither of which are continguous with the state, nor particularly interested in sharing a national identity with each other.

In short, a state is about turf, and a nation is a people, and you need them both to look similar on a map to make a nation-state.

I've heard Brussels as a stand-in for the EU.

I've never heard Belgium as a stand-in-for-Brussels-as-a-stand-in for EU.