No, you will be able to book long before 27 March.
What they are saying is that you can only book flights departing before that date, which will mark the end of their winter season. I would assume that the following day will be the beginning of the airline's summer schedule and I would expect those seats to be bookable in the coming weeks.
I am sure that when the airline responds to you, it will be with a date, letting you know when exactly bookings for Summer 2010 can be made.