Cis means 2nd class person who is lower in status to biological males.
This is why cis is not used to describe men.
That's why Rachel McKinnons rant was "you are terfs, you are transphobes, you are cis"
They were asserting their superiority and making a point that woman were not equal. Women who say they are allies and are happy to call themselves cis should take note of McKinnons revealing outburst
That's why the accusation of cheat hurts most. McKinnon believes that asserting their dominance is legitimate. That's it pure and simple. They think they are a better class of person who is superior and should be rewarded for and is entitled to their superiority.
Cheat cuts through that most
Also see McKinnons recent comments on toilets.
McKinnon thinks they should have more rights and entitlement to toilets and other facilities than other people do. Certainly more than minority religious groups who culturally will be forced out of certain activities and places completely having the nerve to call them bigoted in the process rather than recognising the very real barriers those people have.
McKinnon is an over privileged white supremacist. And certainly does not have the interest of any woman at heart. Not unless they are 'woman supremacists'.