When the banner fills the screen, the 'X' is the unconditional opt out.
You would think so, but if you read these notices carefully a lot of them will say by either hit "Accept" or closing this pop-up you are consenting. So by hitting the 'X' you are unconditionally opting in.
Unless your talking about the 'X' to close the tab at which point, yes it is
Start talkin to some lawyers then, be the change you want to see, etc.