It's more than political. In speaking Flemish is to Dutch as UK English is to US English. In writing however there is no difference in spelling, but there is a difference in word choice.
Now, being from Belgium, even within that small part of the country where everybody is supposed to speak Dutch, I genuinely don't understand people from near the coast, which was about 150 miles from where I used to live.
Well yes, the dialects are very distinct linguistically, but what is often referred to as Flemish is the Dutch "tussentaal" aka "verkavelingsvlaams"[1]. That's not really a language per se, it's a regiolect of the official Dutch language, itself a Dutch variant of the Brabant dialects. The Flemish Dutch is usually used a lingua franca because the official Dutch otherwise sounds too formal (and native dialect speakers are foreign language speakers of Dutch). If I was to nominate a regional language for recognition it would more be the regional dialects like Brugs, Gents, Antwerps, Brussels Vloms, Hasselts, etc.
What I find interesting is that the differences in Flemish dialects make them much more distinct than what would normally call dialects. There are significant grammatical difference beyond the usual vocabulary differences. For instance, coastal Flemish conjugates yes and no[1], Limburgisch is a tonal language.
Now, being from Belgium, even within that small part of the country where everybody is supposed to speak Dutch, I genuinely don't understand people from near the coast, which was about 150 miles from where I used to live.