That should ne in addition to affordable housing, and those contributions are set on a tariff basis normally rather than a % requirement that affordable.
The threshold for a major site is 10 or more in the NPPF. The developer just needs to pay less for the land if they can't meet the Affordable Housing requirements and not hide behind creative viability assessments. It is clear some LPAs are performing far better than others in this area, and despite the Affordable Housing rhetoric MCC isn't good in this area.
Not having an up to date Local Plan doesn't help MCC, as viability is now tested at plan making stage, but the path of least resistance is not challenging developers.