Is my understanding correct, or is argument providing always offtopic?
We're not here to provide those reasons or arguments, we can help with presenting the reasons or arguments convincingly. Remember, interpersonal skills are behaviors you use to interact with other people well. Going by that definition, coming up with the arguments isn't an interpersonal skill, presenting them in a way that others may be more accepting of them is. So again, we're not here to tell you what to say or come up with points to argue. This has been like that for a while, quoted from my answer to the first post you linked in your question:
As sphennings said, 'How can I convince someone to do X' is a bit problematic/poor fit for this site, in that it's often not clear whether they want arguments to use or want to improve their skills. To me, right now, it isn't clear to me if the OP is actually looking for help improving their Interpersonal skills, or for arguments to use to convince their brother. The first one is on-topic here, the second one not.
Another relevant discussion may be Should "How to respond to someone when they ask 'How come you're still single?'" be reopened?, Catija's answer there compares coming up with arguments to tell your parents to 'give me teh codez' questions on SO:
If the goal of a question is "tell me what to say" or "re/write my email for me" - we should not answer these questions. These are the equivalent of the Stack Overflow "Give me teh codez" questions.
You can come up with as many arguments for someone to make their case as you like, but if that person doesn't know how to present them, those arguments will not help them. Just like giving someone teh codez doesn't teach someone the skills to program anything else, or recognize mistakes in the code they're given, giving someone the arguments doesn't teach them to be convincing or recognize when they are presenting the arguments wrong.
Is there any other nuance I am missing?
Perhaps one nuance you're missing is of age and beta site scope changes. The question on main that you linked is older than the meta discussions linked above. It was written very shortly after the beginning of the site, that it isn't closed yet is probably a result of people not going back to hunt down old questions and close them. But it certainly wouldn't survive were it posted right now.
Another nuance: A question that doesn't include the arguments/reasons someone wants to present, and the problem they have with how to present those arguments may still be about the behavior needed and not the arguments. That's why I closed e.g. this one for 'lacks details or clarity', not as off-topic: Without the details and clarity even I can't know if the question is on-topic and just lacking these details, or off-topic because it's asking us for something that's not an interpersonal skill at all.
I do often mention that clarifying in a specific direction will not improve the question but only make things worse. I mostly do so because people will get mad if you ask them to clarify, then tell them their clarification made the question off-topic without explaining that first. While I generally dislike giving away the 'correct' answer like that because it more often seems to lead to very desperate attempts instead of good questions, it's the polite thing to do.
But it's another nuance that seems to have made this more complicated: People now seem to think the question was closed for being off-topic because my comment mentioned that coming up with arguments is off-topic, while the question is closed for needing details or clarity, something that either confirms the post was off-topic, or that makes it an answerable IPS question.