To quote this question, and the article it refers to:

INERSECT returns "those distinct values that are common between tables"

I have never seen a situation where:

SELECT DISTINCT a.x

FROM a

INNER JOIN b on a.x=b.m

would not return "those distinct values that are common between tables" either, regardless of the uniqueness of a.x or b.m in either query.

I agree that the execution may be completely different in either case.

Chris