In fact my post was probably a bit hasted. I can imagine a single masterclass with a great pianist can profoundly influence your way of thinking, but does that make you a pupil of that musician? No… Similarly I think some of the greatest influences on young musicians today are not just their teachers, but recordings they hear by Horowitz, Richter, Cortot, Hamelin or whoever they resonate with. Does that make them pupils of these pianists? No…
New hypothesis: a teacher is someone you’ve formally worked with on repertoire. Basta. Regardless of whether you think you learned anything or not you did study with him/her. More interesting, or at least equally interesting, is perhaps to talk about influences rather than teachers.