She knew what she was doing. She had the option of alluding to or describing what was in the letter. It could have been damning to Sessions but still within the rules, and she didn't take that option. She understood what the consequences would be and can't pretend that it was a surprise to her.
The optics aren't good. Telling a woman to effectively 'sit down and shut up' as she quotes Coretta Scott King isn't a great look, regardless of the context (and I concede that the context matters here, just as I acknowledge that a great many voters will never hear it). Also, it makes Warren into a martyr, breathing life into her obnoxious heroine complex.
Maybe McConnell wanted to feed her martyr complex. Maybe it's part of a strategy to make the Democrats even crazier than they've been. It could have been part of a strategy to boost her as a possible presidential candidate -- a candidate who would be easy to beat in 2020.
That might be it. It's hard to imagine a worse leader than "centrist" Hillary though. Many Dems believe that a younger version of Bernie would have beaten Trump.