The country is split linguistically between Dutch speakers in the northern Flemish region, and French in the southern Walloon region.
The Flemish region's television stations and cinemas run mostly subtitled programs (really, anything outside of children's shows and content made in the region or in the Netherlands).
In the French-speaking areas, the media companies and cinemas buy their content from France, where the vast majority of it comes overdubbed.
As a result, the Flemish have a much stronger command of English than their French-speaking counterparts.
That said, the French-speaking Belgians, at least in my experience of living there for a long time, speak English much better than the French.
A lot of that has to do with how international the country is, where Brussels not only hosts many international institutions, but also a lot of companies have their EU HQs there. Many people need to speak English to get jobs with these firms, so it drives up the English competency among the local population.
Your theory definitely has some merit.