SQL and other declarative languages can be frustrating to talk about because what you tell it you want and _how_ it does the work isn't supposed to matter. So there are two questions that you might answer with either jvsn's answer (how should you imagine it works to understand how to use SQL?) and your answer (how should you imagine it works to figure out why it's slow?). Someone learning SQL is probably asking the first question, somebody debugging performance is probably asking the second one.
So this is a weird criticism because neither of you is incorrect, it's just that different audiences require different kinds of detail. If she instead wrote the answer to your question then someone else would inevitably come along with a comment nearly identical to yours except it would say "well that's just implementation details but you should instead imagine that...".