In the question you sited, the user is simply asking for clarification about your answer; however, there is nothing that requires you to continue answering his questions. There is a chance someone could come along after you and provide a more comprehensive answer that gets accepted because it responds to all his concerns
Although it says "requesting clarification" is OK, where do we draw the line between clarifications and asking a new question altogether?
It's somewhat subjective, but I usually consider it when the questions in comments start creeping outside the original scope of the thread. For instance, if someone asks a question about how the Cisco ASA handles TCP window-scale options, and then in the comments they ask about the PPS throughput of a router attached to the firewall. The router has nothing to do with the original question, and in most cases, it has nothing to do with processing window scale options... that clearly should be a different question.
However, let's suppose the followup question in the comments was about how the TCP window-scale option is handled in a Palo Alto Firewall... it's debatable whether you should require then to ask a different question, particularly if you already know the answer.