There's no solid evidence for your claim either - it's just our opinions.
Everyone keeps talking about the center but that assumes several things: 1) the "center" is an actual position people hold 2) people in the center are a significant population of the voters compared to the non-center 3) the center people wouldn't have voted Bernie even though the alternative was Trump 4) Bernie wouldn't have made in-roads with the center during his general election campaign.
Bernie tends to do well with voters when he talks about his policies. I think it's more likely Bernie would have moved more people to voting for him than Biden did. Bernie would still have the anti-Trump vote plus he'd have a plan and policies to talk about in the campaign. Plus while he caucuses with the Dems, he's still an independent, which gives him an appeal as non-partisan and as an outsider that I think is underappreciated.