Science does not require an unbiased sample from a normal distribution to work. Bias is a technical term that the field of statistics is very comfortable working with. Scientists can also often get good results out of biased inputs.
538 has corrections for bias already. They seem to have worked in this instance - I repeat myself but: massive surprise, Biden still president.
You are pointing at evidence that 538 correctly called 11/12 races using statistics, and their confident call on a Biden president withstood a 4-7% swing (!!).
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.
538 has corrections for bias already. They seem to have worked in this instance - I repeat myself but: massive surprise, Biden still president.