AFAIK, guarantee in the sense "it is our ambition to provide full flat on these routes, but if we fail, we'll fail'. I haven't seen any promises what they will do if no full flat is available at any given point.
You should call AY and ask (not that you will get a better answer but just to make someone realise that if you call it a guarantee, there need to be a "pay-out" of some kind in case of failure)

OTOH, they have more full flats than they have routes with guarantee, so maybe in reality it will be very rare to fail on this promise.