

Yes. This is often true. But Flemish and Dutch are far far closer in linguistic distance than dutch and german.
And they are completely mutually intelligeble. Unlike Dutch and German, (which I prefer to call hochdeutsch, since german is a nationalist contruct that erases many other languages spoken by peoples living in Germany-Switzerland-Austria.)
Like here we get a distance between Flemish and Dutch of 5.6, that’s the lowest I’ve ever seen with this tool.
While 13.5 with Dutch and German.
Compare that to French and Occitan, Occitan is a Romance language in southern France, which got erased and often claimed it’s just “part of french”. The distance between them is 20.
Edit: Playing round a bit more with the tool, Your point is proven. The distance between Dutch and Afrikaans is lower. Only 2. Yet that’s considered different languages.
The tool measures distance with vocabulary. Afrikaans may be closer in vocabulary but pronounced very differently (since there’s way less cross talking since it’s so isolated), which would make it harder to understand to a Dutch speaker?