2 years ahead is a long time to be making a commitment for an airline. Between now and April 2023, ie the main summer booking season, there's a huge amount of information to be gained by analysing customer behaviour patterns - ie where pax use their credit cards, as well as looking at how the economic and political situation develops. In return for the cashflow, Jet2 is committing itself (or risking annoying pax by making changes to their booking)
Does the winter cashflow outweigh these risks, constraints and other gaps in knowledge ?