I’m not sure I agree with Carville on the Nelson part. Incumbency is a powerful staying force. Trump won FL in 2016 by 100,000 points, but IIRC Rubio got elected by 700,000. We should expect incumbents to stay as a general rule. That’s why I predict relatively few Senate flips compared to others (but also relatively few House flips).
I read it as Carville saying if Nelson doesn’t win, that Democrats should turn out the lights — the party is over. Essentially rating the incumbent advantage as you do and calling a flip there in the east a bell weather indication of a bad night for Democrats.