Polling well against Trump is no great accomplishment.
Just ask Hillary Clinton.
Most candidates poll fairly well against Trump up and down the range. (Forget the entire conversation about bias which is a whole other issue.)
But polls measure hypothetical candidacies and abstractions. And Trump performs well in real races in the real world even as he tanks in polls.
It's no surprise that he polls badly against Biden. But the Biden that he's polling badly against is a hypothetical candidate. Trump, as a hypothetical candidate, had no shot against Jeb Bush, let alone the rest of the field back in June 2015.
The actual campaigning changed all that.
Trump doesn't necessarily win by improving people's opinion of him. That's politics as usual. He wins by breaking down other candidates and transforming the entire field around his ideas, issues and personality. That's the code no opponent has been able to crack.
Polling at this stage wouldn't be very meaningful in a conventional campaign. And this won't be one of those.
Biden, circa June of 2019, is not going to be the actual candidate in the race. The campaign will change things. Dramatically.