I've read the letter too. It was a bit disingenuous of dusty to avoid mentioning that her partner is a councillor, but apart from that she was commendably open about her own business interests. It's hard to see what all the fuss is about, and there's certainly nothing 'political' about her post.
Meanwhile, there is an interesting debate here between dusty, who says 'I would not be surprised if there were just as many people who support a supermarket as there are who came out to march against it', and Miss M, who writes
..we may be able to enlighten you on the very strong opposition to a supermarket on the Hibbert Lane site.... come to the MIA stall next week and hear first hand how strong the opposition is to a supermarket development is on the campus. Where on earth you have got information that there is not such strong opposition is also quite alarming ...
This issue is one which might usefully be investigated more systematically. At the moment, AFAIK, there is no reliable evidence for what the majority view is, one way or another. Maybe some local body which has funds (Marple Business Forum?) might be interested in commissioning a proper survey from a reputable research organisation?