It’s an invented prefix that they are using because they don’t want to use the word “normal”. Because, you know, using a term like “normal women”, as opposed to “trans women”, would make you think there is something NOT normal about the “trans women”, and they can’t have that. So they invented “cis” instead, to try to avoid people noticing the obvious.
It is not an "invented' prefix.
It is a perfectly real and historical prefix that has been hijacked.