Belgium is fucking weird, man. Look it up; it's not a real country, or rather it wasn't before it was drawn up in the aftermath of the Napoleonic to contain France.
Belgium in that sense has a bit in common with the entirely fictitious nations drawn up in Africa and the Middle East by the Colonial Powers and the victorious Allied powers after WW1 and WW2.
My understanding is that it's a belief that is widespread among the Belgians themselves, and that many feel that it is inevitable that the country splits in two.
In particular, I remember an interview with Vertonghen a number of years back, when they asked him about the great amount of Belgian talent coming through the Premier League, and in particular how his relationship with Eden Hazard was now that they were playing for rival clubs, and his response was basically "I don't know Eden very well at all, we don't even speak the same language. He's basically French while I am Flemish."