The media released a baseless smear of bernie over the last few days. Members of warren's team leaked a story to the media (almost certainly with Warren's blessing) that bernie had told her in a private meeting that a woman couldn't win. They then spent the whole day the story came out refusing to comment fueling the media to keep reporting it. Warren eventually came out and said it was true. The point of all of this is to try to smear Sanders as some kind of chauvinist.
Sanders has a long history of fighting for women's issues. He tried to get Warren to run in 2016, and only ran himself because she refused. There are also videos of him confirming he believes women can win the presidency dating back to the 80's.
To run this attack on Sanders smacks of desperation. And even worse, she is attacking the person she has the most in common with while letting Biden, a man who has made TONS of terrible decisions, statements etc, completely off the hook.
She can't possibly beat Sanders on the issues because he is the most trusted candidate on most of them. She can't beat him in popular appeal because he has the highest favorability rating among the candidates too. So she takes a cheap shot that cannot possibly be confirmed or disproven to try to undermine her closest ally.
I think this desperate play is bad for her campaign.