The article actually suggests two explanations for the flights: acting as space-holders for the valuable LHR slots or allowing BA to position aircraft to avoid inconveniencing customers, who otherwise would be disaffected on the return segment by flight cancellations on the outbound due to crew shortages.

I'm not sure how its author came to the conclusion that the former reason and not the latter is the true explanation for these "ghost" flights. Which is not to say, of course, that BA may lack an ulterior motive for running empty transatlantic flights. However, would BA really be vulnerable to losing a specific LHR slot if it occasionally was unable to offer service on a particular day for some reason whether due to a crew shortage, mechanical problem, or whatever?