Not sure. Someone may have jumped the gun on the basis that it is no longer bookable after that date. But neither is LDY to STN although strangely STN to LDY is bookable.
As far as I know there has been no announcement one way or the other. Could Ryanair be hedging their bets over Easyjet's move to BHD on the Luton route or are they still loading the summer schedule or are we awaiting a bigger announcement which will affect the availability of aircraft to operate the route (ie: a base).
As the website currently stands it is very odd. Maybe they just haven't noticed!