Also that’s really solid advice for what to listen for when choosing a clarinet

Nothing he plays uses the sixth string, the low E string, for a sounding note. He is either having his thumb on it because it is comfortable, or for dampening it so that he can just strike all six strings with those 16th notes (or both reasons). Notice, in the chord diagrams that occur, the "x"'es over the E string, and occasionally also the A string, meaning the string(s) should not be played, or they should be dampened. In the case of this 16th note funk strumming hitting a dampened string, rather than actually avoiding hitting it, is just fine. Or even preferred. You want that percussive effect!