In order to extend Eurostar service to Amsterdam from Brussels, it will be necessary to launch Eurostar dedicated platforms in Rotterdam Centraal, Amsterdam Schiphol, and Amsterdam Centraal to enable border check before proceeding to the platforms.
But, according to the article, Eurostar trains are scheduled to run earlier than provision for border inspection in those 3 stations.
So, passengers going to London from Amsterdam or Rotterdam will have to change trains at Brussels to go through border check.
Actually, it seems to be the same as current situation that passengers take Thalys from Amsterdam or Rotterdam to Brussels and change trains to Eurostar.
I think Eurostar should begin extensive operation after preparation of border inspection systems.