There are already DEMUs▸ that have been in service, so you can't really redefine them.
Why not? I'm only ignoring a historical anomaly. In any case, the question was more about why it was shortened to
EMU▸ in the pathing instruction.
On its return it's "Timed for 75mph max/Pathed as Class 150/153/155/156 (Sprinter)
DMU▸ ". Maybe that's why it was early when it went past just now. More seriously, I vaguely remember that the only simple way to set up a one-off oddity like this is to copy something that already runs in the area it starts from. That may be the real, if more prosaic, reason.