True. The Dutch did have some really good soldiers. But not many. (They did not maintain entire armies of them, that is.)

Also, consider that this is about:
1) A strong leader - with a certain charisma.
2) People who (at least the Dutch under his command) didn't exactly agree with the French overlordship of the Netherlands. In fact, we are talking about the Orangists (Orangisten, named so after the family of which all Stadhouders (Sterwards) were descedants - Oranje) or Oranjeklanten. And those people were people who ended up in the army and navy (having no other way of earning money, being the lowest social class). Add to this that the Oranjes (the family) had the reputation of defenders of the masses (as opposed to the rich merchants who would exploit them).

So I would say that they had a rather strong incentive to fight for the Prince - and to keep fighting at all odds. Especially since the Netherlands were now officially under French overlordship - which hadn't exactly gone down well with the upper and middle classes either, even though their revolution had been supported by the French and they had relied on French foreign policies to remain in power since 1794. (Most notably because the Dutch had to pay taxes to finance the French military.) So there you are: 1) a feeling of having the opportunity to aid your country & fellows 2) a feeling of power by being united under the banner of a strong & determined leader.