This happened right after Bernie won the South Carolina primary.

Right after Bernie won the Nevada primary and shortly before he swept California. Biden won South Carolina in a landslide, thanks to a coalition of Atlantic Coast conservative democrats throwing their corporate lobbyist fueled and Pentagon endorsed support to the most reactionary candidate in the race.

Warren, also at the meeting, stayed in the race to split the left-leaning vote.

This gets overstated. Warren got her knife into Bernie's back when she accused him of psychically tanking the Hillary campaign. But she had next to no impact on the Midwestern primaries, where Biden trounced Bernie. That was much more about COVID depressing turnout and all the other midwestern candidates dropping out.

Previous to this meeting, Bernie had a solid lead.

He had a few break out states on the West Coast, but he wasn't yet polling above the 50% line. The Obama endorsement combined with the Klobacher/Buttigieg dropouts consolidated support behind Biden in the Rust Belt. It likely would have been a horse-race into the convention, but it wasn't a sure thing.