I don’t actually think that statement is true. Polls are notoriously wrong all the time, and the members of the Democratic Party requesting he drop out are doing so because they’re afraid to say otherwise. I’ll give you an example. Members of the Republican party notably were very critical of Trump until they realised that to continue in the political sphere they were going to have to get along to get along when it became clear he was the front runner.
The members of the DNC who are asking for Biden to step down are doing so even though they know that no other possible candidate is likely to win. They did the same with Bernie Sanders in 2016. They aren’t worried that he’ll lose the popular vote. They’re worried he’ll lose Electoral College votes because some of them are apparently still on the fence. If they wanted to put forth another candidate they would have already. Who have they put forward to put on the ticket? No one. Because they don’t have anyone else either. For them it’s a no win situation and they’re scared so they’re trying to cover their own asses.