For future reference, this is precisely the problem. It doesn’t matter if you expected your replies to be removed, because it sounds like your replies also caused more off-topic posts since you had to send more than one.
Additionally, it is a waste of time to flag many off-topic posts in the same thread. If a flag brings a thread to our attention, then the moderators will read the whole thing in context, and can make decisions on all the offending posts without them all having been flagged. The only exception might be if the posts contain pure spam or illegal content, since flagging them might preemptively hide them, but if they don’t absolutely need to be immediately hidden from the community (off-topic comments don’t) then every single one doesn’t need to be flagged. I don’t think this was the biggest problem, replying was the biggest problem, but it doesn’t help.
I understand your points though, we can update our guidelines to be more clear on both of these points.
If you can send me a link to the DM you’re referring to I’d appreciate it, otherwise I will seek it out. Thanks for your feedback.
Edit: to be clear I am not yet familiar with the specific case you’re referring to, but I think I can tell what the problem was only from what you described.