It's depriving X of impressions that they could monetise, no? Xcancel doesn't have to make money itself, it just has to impair the rights of the copyright holder. Otherwise piracy would also be legal as long as it were non-profit...
Which is in a few jurisdictions, or at least is not prosecuted if it's for personal use. Also, according to your definition, the creator of uBlock Origin or any other adblock system should be sued in the same way, because they are depriving $ADS_CORP of their precious impressions.
i dont think the actual copyright owners care, given they put their content onto a vaguely public view where they aim to get the most traffic to something else they are doing