This article from Rail Journal states:
Customs agreement reached for Amsterdam – London Eurostar services
STATE secretary to the Dutch Ministry of Justice and Security, Mrs Ankie Broekers-Knol, informed the country’s parliament on September 25 that an agreement has been achieved between the Netherlands, Belgium, France and Britain on customs and security arrangements for Amsterdam – London Eurostar services.
Direct services will begin operating between Amsterdam Central and London St Pancras on December 15, when the 2020 timetable comes into effect.
However, there has been no press release from Eurostar on the subject, and the Eurostar website continues to show only trips with a change in Brussels.
I fear they may have missed the deadline for the 2020 timetable change, so this would probably push things back to the next timetable change, probably the summer service which should take effect in June. Eurostar only shows trips up to May 13th, so it's too early to tell for sure.
The Bahn.de shows direct trains on some dates (from March 31st to May 16th, with some of the trains not running on Saturday and/or Sunday), but does not allow booking them.
The Oui.Sncf site shows the direct trains but states "Train non ouvert", which means the train cannot be booked yet (but you can subscribe to an alert to be notified when they become available, though I'm not 100% sure that works in this specific case).
My guess is that the trains are planned to run as direct trains, but they're not quite sure if they will actually be able to run as such, depending on completion of works in Amsterdam CS and/or Rotterdam CS, and won't allow booking those trains until they are sure they will run as planned.
Strangely, this other article from Rail Journal, published later than the first one, states
NETHERLANDS Railways (NS) has announced that the frequency of London – Amsterdam Eurostar services will be stepped up from three per day currently to four per day in 2020 and five per day in 2021.
However, the gradual expansion of the service will only happen if a four-country treaty, involving Britain, France, Belgium and the Netherlands, is signed regarding border controls.
NS says it hopes the direct Amsterdam – Rotterdam – London services will start this spring.
Not sure whether reporting is accurate, and whether the issue is the agreement have been "agreed upon" but not actually signed, or if it's just pending completion of the required works in Amsterdam CS and Rotterdam CS. Given the linked tweet below, probably just the latter.