You are correct: I neglected to mention "on-attack" effects. As you say, the act of casting your Basic Attack will trigger a select number of effects (e.g. Pix). This is just an on-cast event. Casting an ability like MF or Ezreal's Q does not trigger attack effects. My original comment was more about the nature of a basic attack as well as basic damage, but I agree that it is worth stressing that attacking is distinct from dealing an instance of basic damage.
It is worth noting that some "attack modifiers" are released with the incorrect flags/even hooks. For example, Ashe's Q and Xin Zhao's Three Talon Strike both (at one point) didn't count as "attacking" for the purposes of things like Pix or Guinsoo's (because they replace the standard Basic Attack ability but weren't set-up to behave exactly like a Basic Attack ability). Some effects, such as Lucian's innate, count as "attacking" multiple times.