Yeah, "terrorist" is kind of a floating designator. The YPG should be distinguished from the PKK in this instance. The UK takes occasional erratic action against the YPG: https://www.opendemocracy.net/en/can-europe-make-it/terroris...
I'm not saying it is terrorist organization or not, but it's inconsistent to have one of these organizations on the terrorists list and others not, since they all share the same ideology, goals, and tactics.