I thought that 2tph to Knutsford was a franchise commitment too, while 2tph to Buxton wasn't.
2tph to New Mills Newton is a franchise commitment, 2tph to Buxton is not.
Looking at current usage for stations between Woodsmoor and Buxton and for stations between Navigation Road and Greenbank the busiest is Hazel Grove with around 700,000 journeys per annum, the next busiest is Altrincham with around 570,000
rail journeys per annum, followed by Knutsford with around 510,000, then Buxton with 331,000. However, even then Buxton's figure is lower than the combined figure for both stations within the boundary for the town of Northwich (Northwich and Greenbank.) Given there's a DMU shortage Northern's plan makes no sense - Hazel Grove can be served by EMUs while
in the unlikely event that there's high demand for Hazel Grove to Buxton then a half-hourly DMU shuttle from Hazel Grove to Buxton might be more appropriate as high demand for Buxton to Hazel Grove would mean low demand for Buxton to Manchester (not that I think that is actually the case.)