Quite possibly.

The epidemiological studies, as I understand it, make the association between claims of flossing and improved tooth health unambiguously exist (though not huge). HHS didn't analyse them and find them too week, exactly; they simply want controlled studies for this purpose (for good reason, of course). Nonetheless, everything we know makes it sound like flossing is at least a little effective.

Whether the effect justifies spending minutes every week, who knows.