It was already possible to write CSS that adapts to various screen sizes before CSS became a privacy issue (ie. before CSS 3); except it was called regular CSS, not responsive.

My guess is the difference between "regular CSS that adapts to screen size" and "responsive CSS" is that the former only has a single set of rules while the latter has different CSS rules that get enabled/disabled based on screen size.

Conditional rules -> different content gets loaded -> server gets notified of what rules are enabled -> fingerprinting