This is correct. Bonaire, Saba and St. Eustatius are "bijzondere gemeenten" (special municipalities) of the Netherlands and thus technically part of the (European) Netherlands. Practically there are some differences and it's not like I can just up and leave the European Netherlands and move to Saba. Also they don't use the Euro, and some special laws apply instead of the regular Dutch ones.

Curaçao, Aruba and and St. Maarten (Dutch side) are consituent countries of the Kingdom of the Netherlands, meaning they are different countries that have the same head of state but are otherwise mostly self-governing.

So yes a flight from AMS to BON is technically domestic, but thankfully KLM considers it long-haul. AMS-CUR is a bit more muddy because while it is a separate country, a similar situation exists if I were to fly from say GLA to LHR (Scotland to England) and that is definitely considered domestic. Though again KLM considers it long-haul due to the sheer distance involved so the point is kind of moot.