I think you are right. I am mixing up the 12th with 2024 however it was not until Bernie dropped out that everyone started calling Biden the winner.
By March media was still calling Biden the nominee and you can find articles claiming Biden as the nominee by March 17th just with a quick Google search. Washington Post called it by March 15th in an article I can’t read from a paywall.
After Biden won South Carolina (a single state) it was already being called as his win and by March 3rd most other candidates dropped and fell in line with Biden creating chaos in super Tuesday polling as voters were told their votes had been pointless.
I agree with you that if you look Biden didn’t actually get the delegates needed to be the Nominee until June and that Bernie Sanders didn’t drop until the 8th of April and his campaign was struggling at that point.
But that divide between what actually happened and how it’s recorded is part of my point.
A large amount of effort was made to push people into a specific option and while it “worked” it does not mean it didn’t come with a cost of voter engagement.
Years down the line I will wonder why I saved this comment but I hope it will still make me giggle like an idiot then too.