If I understand it correctly, they're arguing that any unauthorized "modification of the computer program" (i.e. the web page) is a copyright violation.

This wouldn't only affect adblockers... this would affect any browser feature, extension, or user script that modifies the page in any way, shape, or form... translators, easy reading modes, CSS modifiers (e.g., dark mode for pages that don't have it, or anything that improves readability for people with vision problems), probably screen readers...

This would essentially turn web browsers into the HTML equivalent of PDF readers, without any of the customisability that's been standard for decades...