It's actually none of the above. AOC ran 5% behind Biden in her own district. So did Omar and Katie Porter.
Generally, further left people are able to primary themselves into very blue districts and then they run 5-10% behind was a more traditional Democrat would. Which isn't bad in and of itself, as it lends progressive voices in Congress, but it isn't a winning strategy in more average districts.
Biden outperformed basically every down ballot candidate of any persuasion, so all these hot takes are basically fucking garbage. If you want to see why Dems lost house races, look no further than Robbie Fucking Mook's worthless ass. Put Stacey Abrams in charge and this shit looks a lot different.