Update February 4th, 2020:
Eurostar will begin direct commercial services from Amsterdam to London in April, the cross-Channel rail operator has announced.
The route was launched in April 2018 but passengers travelling to the UK have been forced to change trains in Brussels to complete passport checks and security screening.
Eurostar was assured that the UK and Dutch governments would agree to allow border procedures to be carried out in the Netherlands by the end of 2019, but progress has been slower than expected.
The Department for Transport said the UK Government is “finalising an agreement”.
Commercial direct services from Amsterdam will begin on April 30, with trains also serving Rotterdam from May 18.
There are currently three outbound services a day from the UK to the Netherlands.
The direct inbound journey from Amsterdam to London will begin with two daily services, with the aim of increasing this to three and then four.
[Rail expert Mark Smith, founder of Seat61.com] described the firm’s offer of city centre to city centre journeys in three hours and 50 minutes with a check-in time of just 30 minutes before departure as “very competitive”.
Eurostar said tickets for the new direct services will be available from February 11 and will cost from £35.
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.
Summer 2020 note: due to COVID 19 the start of the service has been delayed and is now expected 'late in the year 2020'.
Edit 26 October 2020:
Today the first direct train did run per schedule.