Though I think it's much more likely that he said something like what she claims, but she's misconstruing it.
'Sometimes I think maybe America is still too sexist to elect a woman president' seems to be completely in the zeitgeist of Democratic beliefs after Trump gets elected over Hillary. Completely in-character for a Democrat to have said and just wait - if Trump beats Warren in the general, they'll say it again!
As opposed to something more like 'I guess women just don't have what it takes to get elected president' which is what I guess we're supposed to believe he said, according to Madame 0.1%. A statement like that seems very out-of-character for Bernie, but socialists believe a lot of crazy things.