After waiting some years and looking into this several times, the answer is that disjointness checking is not supported.
Checking disjointness of Stateflow truth tables?
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Monika Jaskolka on 22 Sep 2014
Is there a diagnostic tool for Stateflow truth tables which does disjointness checking on the decision rules? I am aware of the overspecification and underspesification checking tools, and am looking for something similar which flags/checks whether or not decision rules overlap. Stateflow documentation yields no results.