That doesn't stand up to any sort of scrutiny: DB have been perfectly happy to hire locos out to other operators (e.g. TfW, LNER, Anglia) that aren't crewed by DB. Sounds rather like someone making an excuse up.
The thing is, the sleeper operation is very oddball in comparison with the rest of the GWR operation and really lends itself to contracting out traction supply to an organisation that has more resources to support it. So, someone like DB Cargo........
Yes it means change and possibly changing crewing etc. But ultimately it is the service to the punter that matters and the internal politics around such changes just show what the railway is best at - disappearing up its own fundamental orifice.