He hasn't lost Primrose's vote - he never had it. Some votes are not worth chasing as you will loose more of the principles you stand on, and therefore your core voters, in order to get them. He does not need 100% of the votes and can afford to ignore any extremist views. As I tend to ignore those people who do not argue the political points just attack Corbyn the man most of the time..
What does anyone really care about such attacks. He's flawed, you're flawed, I'm flawed - so what. I want to know what people's politics are, not nonsense about what they did half a century ago. Young people will certainly not be swayed by such tittle tattle.