IS NOT DISTINCT FROM expressions in join condition might actually be considered as equi-join conditions, and RelOptUtil#splitJoinConditions() has support for it. But some other join related functions/rules don't.
One of them is RelOptUtil#pushDownJoinConditions (used by JoinPushExpressionsRule) which tries to push filter expressions below the join, but ends up modifying the join expression in a way which makes identify an IDNF condition impossible later.
For example expression OR(AND(IS_NULL($1), IS_NULL($4)), EQUALS($1,$4)) will be changed into OR(AND($3, $6), EQUALS($1, $5)) which makes it harder/impossible for RelOptUtil#splitJoinConditions() to identify an IS NOT DISTINCT FROM equi-join condition.
This is a variant of