I just read an article from two days ago that said that a bit over 1 million Democrats had voted early in Texas alone. Add the fact that around a week ago 20% of all California Democrats who had been sent absentee ballots had returned them, and I have to believe that a heck of a lot more than 2 million Democrats from Super Tuesday states voted prior to Biden’s SC victory (not to mention prior to Buttigieg, Klobuchar and Steyer dropping out).
Will try to search for a better estimate