I think that's the difficult thing about the concept of privilege. It's difficult to recognise or accept that you have privilege until you realise that some things you take for granted can't be taken for granted by others.
The term "Cis" isn't meant as an insult, merely as a descriptor in the same way as "white" or "non disabled" or "middle class" are. So, I'm not offended by the term any more than I would be by the first two on that list that do apply to me, say.
I suppose the point is, do folks believe that trans people experience discrimination, disadvantage or abuse in society because they do not conform to the "accepted" gender or sexual identity roles? It doesn't matter whether or not you agree with those socially-endorsed roles (like gender) or not. It doesn't matter whether or not you had anything to do with those socially-endorsed roles existing. If one believes that trans people experience disadvantage because they are trans, then the flip side of that disadvantage is Cis privilege.
Just as the counterpart of sexual discrimination, abuse, etc. is male privilege and just as the counterpart of racial discrimination, abuse, etc. is white privilege.
It doesn't mean that just because a person benefits from one type of privilege (e.g. white privilege) that this negates other forms of oppression they experience (e.g. sexual discrimination.) Often here, I've seem working class male members protest that they don't accept that they are more privileged than women, particularly educated or middle class women. Thing is, the disadvantages they experience due to class don't negate the privilege they and all men experience within a patriarchal society. In my view, they often fail to engage with this not because they don't understand the dynamic, but because they don't want to "wear" their male privilege. I also see that alot in the Guardian CiF comments from otherwise pretty right on men who can't wrap their heads around male privilege or the need for feminism.
There isn't a Eurovision style system for totting up points awarded for privilege or points deducted for disadvantage systematically. Also, the experience of disadvantage isn't just "oppression plus," but can be unique at the "intersection" where, for example, sexism and racism meet, or homophobia and classism meet.
So, if one doesn't accept that trans people experience any disadvantage for not conforming to the socially dictated gender and sex roles, then you won't accept that Cisprivilege exists. (In the same way a working class MRA will insist that male privilege doesn't exist.)
If you DO accept that there is oppression against trans people that non trans people don't experience, then you accept the existence of Cisprivilege.
I understand why people don't identify with the term "cis" though. I don't wake up each morning seeing myself as white, or as non-disabled or as an employed person. Those things just "are," and I'm only aware of them when I encounter a situation where someone who isn't those things experiences things differently. I've been on forums where people have resented others pointing out their white privilege, saying they are "colourblind," (which imho is a way of denying white privilege.) The people who refer to them as "white" aren't doing so trying to offend them, but in pointing out their ethnicity and subsequent racial privilege, they may feel uncomfortable and conflicted, and translate this feeling into "offended."
So perhaps thinking about why one feels uneasy/hurt/offended/upset by the term "Cis" would be beneficial.