Sanders doesn't need a campaign for president in order to understand the extent of prejudice in the justice system. Especially with his record on civil rights participation established decades ago in New York City, mayor of Burlington VT, and years in the Senate, that statement does not compute. Because of course it's not the full quote.
In the full quote there are a few things he admits he didn't realize, like the 400,000 incarcerated but not charged with anything. He also discusses learning the specifics on how the experience differs state by state.
Beware of quote-only tweets from journalists without attribution or article.
As for Biden's staff vs. Turner, using population percentages and polling data to tell specific people who they are and what they value is always a bad idea.