Essentially this. Frisia for a long time consisted of a load of autonomous, semi-marsh based town and village communities, which Holland claimed suzerainty over. They had no ability to assert dominance over the Netherlands and even if they had the numbers and the strength, they didn't have any sort of cohesive government etc. It's comparable to saying "what if the Scottish Highland tribes dominated the British Isles in the 1400s?"
The Polish situation was worse in many ways, as the de jure status of King of Poland was stronger than in the Netherlands, yet de facto the Sejm (Parliament) overruled the King time and again. In the Netherlands the King was limited, but at least people tended to listen to them and do what they said, even if they did talk behind the King's back of ending the rule of Kings after the present sovereign at any given time.
Of course, the interesting position in this is that the Stadtholdership was two-sided - the Dutch could do nothing to prevent William's heirs taking up his legal titles (Stadtholder of Holland, Zeeland, Utrecht, Guelders and Overijssel), yet had the power to discontinue the position of the King. It had been discussed on this site before, but if they did that then it's therefore possible for the King of Great Britain to be in Personal Union with half of the territories of the Netherlands independently, and yet exert no actual control over the country itself. It's comparable, if you like, with being crowned Duke of Aquitaine, Berry, Anjou, Normandy and Burgundy yet not being King of France - the difference being in this case, they probably wouldn't be able to just manipulate their power to dominate the Netherlands whereas in France, someone with that many titles could likely storm Paris and be King in a week.