One could always reply to another's reply; i.e. the quote and that doesn't always refer to the OP's question.
If you are replying to an off-topic post that diverts the discussion from the original idea or topic, you are participating in the process of changing the course of that discussion to something that shouldn't be. If the off-topic post gets deleted, so will your off-topic reply to that post.
But that is something we have all done and sometimes the discussion expands in such a way that you forget what the original OP was even about.