Welcome back - SM.
We already know that they cannot meet the certification standard because of their design; what can be shown is an equivalence meeting the more important engine isolation and fire protection criteria and provision of appropriate performance data.
As was stated previously, most (if not all) of the applicable types have already been assessed so the problem is reducing. What was not expected was that some newer type (certificated since the introduction of Appendix C to Part 27) would be submitted for assessment.
Jim