With regard to the sequence test the Hibbert Lane site will be applying for planning permission when the Chadwick Street land is being marketed and hence the application for a Supermarket at Hibbert lane will fail the sequential test as there will be land within the District Shopping Centre that is available for development.
What is worrying is that the College and it's consultants were warned of this by the Council and chose to ignore it. Another example of mis management by CAMSFC Governing Body.
Your point regarding the sequential test is true in the short term - a cynic might suspect a little bit of political influence here to suddenly seek to sell the Chadwick Street asset.
If it is an attempt to try to spike Hibbert Lane, it could backfire in the longer term though - for example if the site was to be bought by a non supermarket user or if the site was to fail to sell because economically no-one could get the site to stack up for development. In either case, it could then easily be argued that the one remaining site available for supermarket retail in Marple had either gone or was a non-starter and thus the sequential argument folds.
I have some doubts as to the Chadwick Street site as a major developed site anway - as others have noted it doesn't lend itself easily to development and could be a nightmare / extremely expensive to deliver. Furthermore, most of the traffic arguments hurled at Hibbert Lane are the same, if not worse, for Chadwick Street.
I wonder if those houses nearby to Chadwick Street with their "No" posters in the window, ever realised that the MIA campaign may could potentially land them with a supermarket in their own back yard