Initially the guarantee was if the flight was oversold in First, Business and Economy you could then buy a Y class ticket and get on - but it was still as tedious to arrange as it is today.

Then when Platinum came out, they ostensibly changed it so that if it was full in only Y you could then do the same thing with the Y class ticket. But I suspect it's more to do with EK and their really poor IT system - clearly not many people are taking advantage of it, so they haven't yet built a system where the seat guarantee, skywards status and seat loads all feeds into the magical revenue system which then will say computer says yes... if it did, they it shouldn't be a problem to book these tickets online, like say, Marriott with their Platinum guarantee - if you log into their website as a Platinum and a hotel is full, a room can be booked, but if you aren't logged in, it says there is no availability.