Subclassing has always been understood to be the specialization of a specific theme under a main class; to use the D&D example, a "Wild magic" sorcerer, is a pure sorcerer who has some additional class features that align with a wild magic theme, it is NOT a Sorcerer with Action Surge (the class ability of Fighter). Now Subclasses can involve things that are similar to the things another class offers, but it isn't ripping their abilities directly. Proper subclassing in ESO could involve a 4th skill line "slot" for each class that would allow you to choose between skill line options that double down on your core class themes. That isn't what we got, what we got is the ability to steal skill lines from another class, to gain that other class's features exactly. That has always been understood as multiclassing.
I'm sure ZOS won't care enough to fix this, but I personally find it kind of annoying. Hopefully it isn't too late.