The outrage is more that a label is being applied to them. They want it to be 'women' and 'trans women', where only 'non-normal' identities get a label.
The application of 'cis' bothers conservatives because it changes the narrative, from people who identify as their assigned sex being 'the default', into cis people just another state of identity with no more significance than the others.
Ah yes, biblical genders: the penetrator and the penetrated.