Exaggerating Biden’s lead might have been calculated to discourage Trump voters, but on the other hand, it might well have the effect of making Biden voters complacent and less likely to run the risks (and inconvenience) of turning out on election day.
There are actual legitimate systemic reasons why pollsters who are lazy and sloppy will get it wrong in the “Age of Trump”:
https://www.youtube.com/watch?v=Wf6yr_McIFs&feature=youtu.be
Pollster leftist bias is just icing on the cake...