538 -- and thus the polls -- were fairly on point, no? A lot of people seem to fundamentally misunderstand how these projections work. The numbers people cite from that site are almost never projected winning margins, they're the share of simulated scenarios (informed by polling aggregates) that saw a given politician win. So the +8 you're referring to likely wasn't talking about the actual margin by which Biden won, but rather the probabilistic share by which he beat out Trump.
That's looking fairly accurate -- if you run the states that have been called by all major outlets through the 538 battleground calculator, we see a 3% chance of Trump winning. Then add Pennsylvania's 1.4 million uncounted mail-in votes (which overwhelmingly favor Biden), Wisconsin's 200k+ uncounted absentee votes, and the fact that Georgia's remaining counties are democratic strongholds... It's kind of difficult to really imagine Trump winning at this point.