That's an opinion, not a fact, and one that depends on who you ask and where. If you go to the Flanders region people will certainly tell you they speak Flemish.
Language researchers can argue all they want about how languages should be grouped, but they sound different, they use different words, and people call them different things. It is simply incorrect to state that people in Belgium speak Dutch. Might as well just expand the group to include German and English (etc) and say they speak Germanic.
Fun fact, the Dutch call their language Nederlands and refer to German as Duits ("Dutch").
The language is Dutch, the collection of dialects spoken in the Flanders region is Flemish. If you're feeling charitable, you can refer to it as a language variant.
Here is the constitution of Belgium: https://www.senate.be/doc/const_nl.html
Articles 2 and 3 define the Flemish (Vlaamse) community and region. Article 4 defines the language areas and specifically mentions Dutch (Nederlands), not Flemish.
> If you go to the Flanders region people will certainly tell you they speak Flemish.
And they would be right. They do indeed speak the Flemish dialect.
> That's an opinion, not a fact, and one that depends on who you ask and where
I don't see how it is "an opinion" if Flemish literally adheres to "Standaardnederlands" (https://nl.wikipedia.org/wiki/Standaardnederlands). Your analogy to German doesn't make any sense here. The Dutch language doesn't adhere to some "Germanic" standard language or anything, and barely has any grammar rules in common with German (even though the languages are very similar).
Language researchers can argue all they want about how languages should be grouped, but they sound different, they use different words, and people call them different things. It is simply incorrect to state that people in Belgium speak Dutch. Might as well just expand the group to include German and English (etc) and say they speak Germanic.
Fun fact, the Dutch call their language Nederlands and refer to German as Duits ("Dutch").