I'd guess a mix of 2 reasons - Most likely is that there simply isn't a suitable electric timing load. The other is that timing it as diesel gives them a bit of slack against timing it as an 86/90 or 92.
Remember the RTT 'Pathed As' field is the NR 'timing load' for planning purposes. Many (most) timing loads do not have timings in the database for all routes and stopping combinations. It is not always a specific description of the individual train's formation. There are many reasons why a train (particularly a train hauled by a loco not (yet) in regular service) would need to use a timing load quite different from the actual traction being used.
