But that's tptacek's original point: what if she saw a "3 of Cups", and said "3 of Cups means that Professor X did Y (in the Conservatory)".

Is that an opinion (her interpretation of what the card means) or a lie (because it's highly unlikely there's any system in which that card could possibly lead to that interpretation)?

That isnt how the cards work, nor could anyone with the most rudimentary understanding of the cards interpret such a thing from any set of cards. I don't know if "liar" is the word i'd use, but anyone making such concrete statements based on a reading is not doing any research whatsoever on card meanings. I am in the camp that the cards are an excellent tool at helping the user examine their situation/self/ethics in an abstract way that ultimately help us better understand our feelings and address issues that we may be ignoring. It does seem rather uncanny reflecting on previous readings and how they consistently seem to accurately reflect situations and courses of action, but surely this is what i projected onto the cards?