Not unbelievable - it means that Northern have been able to confirm that a bus will be available on the night(s) concerned. Whereas previously it was only planned to operate.
All it means is that they intend to ask their replacement bus provider to run a bus then.
If, as is entirely possible, their provider can't source a bus company willing to do it at the rock bottom prices that Northern are willing to pay, there won't be a bus and you'll have to hope that they're going to put on a taxi rather than P-coding the bus and pretending it was never advertised.
Unbelievably, the resourcing of replacement buses doesn't start until a few days out in most cases.