Fish move. That is an undisputable fact. To claim they migrate implies that it is a repeatable event, yearly, and that they return where they came from, which there is ZERO data to support. Like Chief, a long-term radio tagging study is the ONLY way to confirm that they migrate.
One thing I find funny is that in other parts of the country, the smallmouth don't migrate, rather they school in the deepest holes with suitable winter cover and are caught all winter long by anglers who know how to catch them. I guess if you don't catch any, you can blame it on the mass migration, rather than admit that you struck out or haven't learned how.