Different groups of customers can have on average different preferences. ISP content discrimination could be correlated in favor of the preferences of some groups but against the preferences of others. Even if the dont do this on purpose.

Let's assume that facebook pays a lot of money to ISPs to be prioritized. Meanwhile github/gitlab can't afford to make any payment and these websites feel slugish. Let's also assume that the consumers of facebook are mainly female and the consumers of github/gitlab mostly male. ISPs would de facto be discriminating based on gender, even when it is not explicit.

EDIT: This argument should hold as long as consumers groups according to race, color, religion, national origin, or sex have heterogeneous preferences and if the internet provides more than one type of content to be consumed.