TBH, I think this attack was a triple miscalculation by Warren:

1. Doing this one day before the debate and more than a year after the incident is such and obviously political move that even people who might tend to believe her are skeptical;
2. The attack itself was misguided, as Bernie has a much better reputation as an honest actor than Warren does; and
3. Warren didn’t expect Bernie to be so clear in his response calling it a lie.

Bernie did not respond to Hillary’s lies and obfuscations very clearly in the 2016 primary IMO. Bernie has been rather tepid (again IMO) in his attacks on Biden in this primary up to now. I think Warren was expecting not a “she said/he said” but a “she said, he demurs but timidly and wants to change the subject” that she could then use as a springboard for the “get a progressive woman to the nomination, not a sexist progressive man” tactic that Hillary also tried (and failed) in 2016 against Bernie. Instead this has just highlighted Warren’s well-documented on again/off again relationship with the truth.