It is true, if you are booking well in advance, easyJet is about the same price. The advantage with iDBUS is that you can get the same fare at short notice. For example, I have just checked for tomorrow night, and I can get a £33 single from London to Amsterdam.
Another good thing about iDBUS is that you can choose your seat, and you can see what seats have been booked so far. For the journey tomorrow night, there are only a few seats booked. If they are using double deckers now, that is not apparent from the seat diagram on the website. There are only 48 seats to choose from.
I've used iDBUS between London and Lille, and between London and Paris before, and most of the passengers appear to be French speaking, which isn't surprising given it is owned by SNCF and can therefore advertise through the French rail operator's existing channels of publicity. For a journey from London to Amsterdam, only calling at Brussels, there is no such luxury so they have to find new places to advertise.
I am prepared to pay a bit more to use iDBUS because their coaches are more comfortable than the competition. I particularly like the fact their aisle seats can move sideways by about 80 mm, which gives you more privacy.