Well Flanders is not Occitania, but is bigger than Britanny, Alsace or any other non romance area. At the same time while I acknowledge that Flanders is going to be more French speaking than today, it´s still not quite primary French.
Not now, no. OTL's language demographics have been altered by the existence of Flemish autonomy and vibrant language policy, and even then have had their limits, most notably with Brussels' solidification as a Francophone metropolis and the Francophonization of its nominally Flemish suburbs.
We can't necessarily count on the factors which led to a Dutch language revival OTL in Flanders emerging in this TL. Even without any revolutionary centralization, we could easily have a situation where the attractiveness of participating in a larger France makes a shift to French language popular.
Also what difference does that make when at the end the small minority ruled the country for more than a century? I mean is not like they didn´t force their hands with the policies.
It does, by virtue of minoritization in a larger polity. There were roughly as many speakers of Cornish as there were of Icelandic at the beginning of the 14th century, but because Cornwall was part of a larger Kingdom of England while Iceland was an isolated and self-governing island the pressure for assimilation was that much greater. In a scenario where the southern Netherlands, already run by Francophones, are directly incorporated into the French state, the Flemish will face that much more pressure.
Language is not the same thing as ethnic identity. English is now the dominant language in Ireland notwithstanding the Irish people's sense of its own nationhood. In this scenario, I would not care to bet in favour of Dutch remaining the main language of OTL Flanders.