Early in the Seventeenth Century, people from both Britain and the Netherlands settled in the area that is now New York State. For many years, these two groups competed for power.
The British finally won out. Because the Dutch and the British were enemies at the time, the British invented many slurs that were intended to poke fun at the Dutch.
A Dutch Treat isn’t a treat at all, but an occasion on which all people pay their own way. A Dutch defense isn’t a defense, but a surrender. Dutch praise isn’t praise, but reproach. And a Dutch nightingale is a frog!
Dutch courage is the courage of a person who is drunk, and a Dutch drink is the last one in the bottle. Dutch comfort is the feeling that “things could be worse”. Still common is the expression Dutch uncle, which means a person who scolds and lectures. And to be in Dutch k to be in trouble.
If the Dutch invented their own slurs against the British, none of them has come into our language.
The explanation here seems rather america-centric. The main reason for the English inventing anti-Dutch slurs came from the numerous Anglo-Dutch wars that occurred over the years. A lot of these had to do with trying to achieve hegemony over maritime traderoutes, etc.
Viewing it only from their North-American colonies’ perspective seems to me to be missing a large part of the story behind the slurs.