I found it confusing too, but I think it means that Biden's vote share in the three categories significantly outpaces the percentage of Democratic voters, meaning that Biden is winning large chunks of Independents and some Republicans.
To simplify it further, when you break all this down it adds up to being on track for about a Biden +5 victory, which fits with the polls and by the way would be the biggest win in Florida since Bush 2004.
I may be misreading, but this seems like somewhat circular logic, as the estimates are based on a poll showing Biden +5 with registered voters. So, the Biden +5 result you get by doing the math is only really as strong as that poll, not so much the very concrete data we have on the early voting numbers from Florida.
