Necro-thread, but the reason WHY the best Indonesian food in the Netherlands is (arguably) in The Hague is simply that after WWII when Indonesia gained independence (i.e. from ~250 years of Dutch colonialism and a few years of Japanese occupation), the bulk of the population of mixed Dutch-Indonesian descent fled, mostly to the Netherlands, with a plurality ending up around The Hague in particular. It's a big population of a fairly small (~500k) city, and The Hague is self-sustaining as a center of that culture (see the Tong Tong Fair as an example.)

Now, it must be said that Indonesia is an exceptionally large and diverse country - the culture from island to island varies significantly (and there are tens of thousands of islands!). So the idea that there's just one kind of "Indonesian food" is as wrong as the idea that there's one kind of "Chinese food" - and what you'll find in NL is influenced by this history of migration. The predominant Indonesian cuisine in NL is Javanese specifically, sometimes by way of South America (!) as large numbers of Javanese were transported to Suriname to work on sugar plantations (back in the time when both were Dutch colonies.) In the years after Suriname gained independence in 1975, there was another wave of migration to the Netherlands for folks with connections and means, and again, many ended up in The Hague.

For me personally, the "best" Indonesian food to be found in NL will always be in someone's home, and will reflect their family's history - but that's not really accessible to visitors. (Also: forget "authenticity" in the sense of the indigenous - that's not the name of the game when you're talking about a cuisine that has evolved over decades of migration and centuries of occupation. If you're getting a 'rijsttafel,' any authenticity is already anchored to a certain late-colonial era. But then again, it's hard to imagine Indonesian food without chili peppers and yet just as with the tomato and Italy, these were unknown there before European contact with the Americas. So maybe the quest for "authenticity" isn't worth pursuing to its logical end.)

In The Hague, I tend to steer visitors to Soeboer or Keraton Damai as well-established and -honed exemplars of (NL) Javanese cuisine. I personally prefer them to anything I've had in Amsterdam, and they're definitely accessible - but are they worth a special trip? Ehh, I dunno. But if you're coming anyway to go to the beach, or the Mauritshuis, or the Kunstmuseum, etc then why not?