Gonna side with Bernie in this whole ordeal. I seriously doubt Warren is the type of person to have such a comment flinged at them and let it slide for over a year while also propping up her primary opponent, and I also doubt Sanders is the type of person to give a fuck about Gender, just that they are a strong enough opponent to face Trump and slowly fix the country. Right now its looking to me like Warren saw the opportunity in this smear campaign to get one over Bernie and chose personal ambitions over country and a united front, and right now that is the thing we need the least of going into the Democratic primaries.
Warren's not the type of person to not shut that shit down when it's said to her face, friend or not. I think she thought it was over, she forgave but it wasn't forgotten. It's in character for Bernie to say something in a tone deaf manor, that's dogged him in the media since '16 - he's not an excellent communicator, I don't think he would have hurt her intentionally. In addition he's a person who would have a target on his back because he is not liked by many people for being an asshole on multiple occasions in politics, being really bad wth how he phrases things and he's more interested in attacking then apologising for fucking up. How can there be a united front when Bernie's campaign is doing things which Warren's camp think are breaking their truce? It's not all on Warren. Personal ambition isn't an issue you have with him when he's an asshole and you can bet he's not going to drop out when it's a choice between him and Warren.