It stops in Rotterdam but only for taking passengers (you can see that if you go to the Eurostat or NS international website and try to book a Rotterdam-Antwerp ticket or so. Formally it is not possible to take the Eurostar between Amsterdam, Schiphol or Rotterdam, no tickets sold for that. But if you sneak out quickly without blocking things for passengers that board with heavy luggage for instance, my guess is you get away with it. As indicated in this thread it is pretty unclear if Eurostart reports back to KLM if people haven't done the full journey.