I'm pretty sure he made that promise already knowing who he wanted. It was almost certainly decided ages ago.
Here's Klobuchar, who knows the score:
https://www.politico.com/news/2020/0...f-color-329301
The reality is that his short-list was likely decided months ago, and the top name has always been the top name. Barring an unforseen development in the vetting, it is likely that the relevant people have known for months.Sen. Amy Klobuchar late Thursday said she personally called presumptive Democratic presidential candidate Joe Biden to advise he pick a woman of color as his running mate, effectively announcing the end of her vice presidential aspirations.
"I truly believe, as I actually told the vice president last night when I called him, that I think this is a moment to put a woman of color on that ticket," the Minnesota Democrat told MSNBC's Lawrence O'Donnell. "And there is so many incredibly qualified women."