Thinking about this, it makes no sense for the reasons you state. What a total waste of precious DMUs when there are plenty of EMUs and more to come.
Clearly they are going to introduce them on the Southport - Alderley Edge route so use them there for less than a year then haul them off? Presumably going back to DMUs under the wires on a route as long as Manchester Victoria to Alderley Edge as well? If it happens, it is one of the most ludicrous decisions I'll have seen in the railway industry to date - and that's saying something.
Indeed, if correct it does seem somewhat perverse, on the face of it, to switch the 769s on to a route where they will make less use of their diesel capability. Especially considering that the additional ex-WMT 323s will be arriving next year and looking for gainful employment under the wires.
One possible rationale might be that it will reduce the number of drivers and conductors who will need to retain traction knowledge on the 769 micro fleet, once Northern has returned the remaining 319s. Thereby simplifying traincrew diagrams and reducing costs.
And the only alternative option might well be for DMUs to work the new Liverpool - Stalybridge stoppers, which would mean an even higher proportion of running under the wires than using DMUs on Southport - Alderley Edge, plus an inability to keep to the required timings between Lime Street and Ordsall Lane. Arguably the larger the percentage of the route that is under the wires, the greater is the justification for using bi-modes rather than DMUs.
Furthermore, we do not yet know what other, related, timetable changes Northern might be planning for December 2020 / May 2021. Possibly the Southport - Alderley service might be split at Manchester, or rerouted via the unelectrified Atherton line instead of under the wires via Bolton, and/or interworked at Southport with another DMU service for which the 769s would be unsuitable (e.g. to Leeds or Blackburn via Todmorden).
