I find really strange that there are, at least according to this poll, so many people who would vote Biden (in a Trump/Biden matchup) but who would vote Trump when he is paired against another democrat, I mean you either like Trump or you hate him, there is no middle ground with him.
I don't think it's at all strange. There are certainly some on the right who don't like Trump for various reasons, but would vote for him to "save the country from being led into Socialism" by someone like Sanders or Warren. Biden isn't perceived as the same threat, so those people are open to voting for him.
Sure, there are some of these voters but
1. They represent at the most 2% of the electorate and when you look at the RCP poll average you will see that Biden leads Trump by 4.5 while Sanders leads him by 2.5, in other words these center right Bidenists are relatively few
2. These voters are largely upper class, middle aged and college educated men, in other words they aren't your average Florida voters.
If I had to guess from this poll (could not find crosstabs) Hispanics, and more specifically Cubans, are making the difference for Biden.