I hasten to say that this is just in one poll--the latest Quinnipiac poll--and of course does not mean Biden would necessarily beat Trump in a real-world election. But it is still instructive. Here's why.
First, note that Biden is the only one of the Democratic candidates tested (Warren, Harris, O'Rourke, Buttigieg, Castro, Sanders) who beats Trump in this poll. O'Rourke, interestingly enough, loses to Trump by a margin essentially identical to his deficit to Ted Cruz in 2018.
So what's the Biden difference here? Comparing this Biden-Trump trial heat to the O'Rourke-Cruz exit poll results (an imperfect instrument I know, but that's what we've got), Biden runs about as well as O'Rourke 2018 among Hispanics, blacks and white college voters but runs a significantly smaller deficit among white noncollege voters (39 vs. 48 points). There's the difference.
Could Biden replicate that (relatively) good performance among white noncollege voters in a general election. Who knows? But whoever the nominee is, that's going to be part of the formula for really putting the state in play.