The inputs it uses to spit out probabilities is known to be bad. Any scientist or researcher who claimed to get valid results from known bad inputs would be ridiculed.
538 has corrections for bias already. They seem to have worked in this instance - I repeat myself but: massive surprise, Biden still president.
The existence of bias doesn't invalidate their predictions. Everyone knows that polls can be badly off target in a biased way - that isn't a new phenomenon.
When they talk about X% chance of Y being president they should be optimising to the outcome, not the margins.