Not saying there was never a poll that had Sanders doing better, but the vast, vast majority had Biden as the leader.
I think we seem to be talking about different things. It's not really about a leader poll.
Polling companies did polls of prospective runners against Trump. Sanders was polling well against Trump (generally 2-3 ahead). Biden was a bit behind that. Since becoming the candidate Biden is now well in front. We obviously don't know if this would have replicated with Sanders, if it would have been greater or leser with Sanders. Given the scale of the Corona crisis it's likely a similar pattern would have been followed. However there is some speculation there.
At the time of the decision though, and I'm being favourable here, there was no discernible or obvious advantage to havign Biden over Sanders. It was purely an ideological decision.