I suppose there are two logical conclusions that can be drawn....
1, Originally A Musical Passing for a Miserable Muscroft was scheduled to be either the first episode of series 24 or in it, and the Lair was to be the Xmas special and all of 24 plus the special were filmed at the same time, and subsequently a decision was made to change the Xmas special to be Muscroft and Lair went out as the new series opener
or
2. It's a forgery by someone who hasnt researched it quite properly and thinks nobody will know enough to spot any anomaly anyway.