Maybe they think RFK will help?
FWIW democrats have had a registration advantage my entire life in NC and it's done nothing. It does not correlate 1:1 with winning. LA, KY have a dem edge yet they lose by huge margins. AZ and CO were examples of an R advantage in headcount.
It's such a massive gap in FL that it would require some anti-Trump Republicans voting for Biden. I got the impression that wasn't the flavor in that state but I'm not the wizard of elections.
As you correctly pointed out Florida is not in the Democrats and Bidens hands anymore. It would require the November Turnout among Republicans to be extraordinarily low and Biden I don't think can bank on that.
At the Book Closing on October 19th 2020 Democrats enjoyed a 134,000+ Registration Advantage.