I would agree with "I don't see how this contradicts Sanders' message or ideals" because it doesn't. But effectively saying 'Look which bigot likes us!' to their 10m followers definitely undermines his message since the message is about inclusion, LGBTQ+ rights, etc.
I think you need to put this in context of who actually dislike Rogan, and who considers him an actual stochastic terrorist.
In the very left sphere, yes, Rogan is a stochastic terrorist. He has enabled and hosted bigots, racists, nazis etc etc. but is this the hill that's worth dying on if it undermines Sanders' chance at actually winning? Especially when this isn't an endorsement of Rogan. Sanders just told Rogan what he believes in, and Rogan said "I like that".
I'm personally starting to feel that "identity politics" and "purity tests" are detrimental to actual change. Actual change comes from fixing class issues with education and more wealth distribution. Yes, we need to call out bigots, but without actual change of the systems behind them, there'll just be another bigot tomorrow created by the system.
With this said, I will not belittle or blame anyone from being frustrated and sad for what this advert means for them. I can't tell anyone who's been actively or indirectly hurt by Joe Rogan and his actions, that their feelings aren't valid; they are.