You have observed a pattern of behavior on HN, and conjectured a plausible legal framework that explains that behavior.
That's great. But, as is going to be the case probably more than 99% of the time that you try to infer the law from how people behave on HN, the rule you seemed to have inferred ("anyone except a lawyer is free to give legal advice in any situation, but lawyers are more restricted") is very much not the actual legal rule.
i suspect part of the error is that your conjecture about the rule implicitly assumed that both lawyer's and non-lawyer's behaviors reflected the same degree of knowledge of, and concern for adhering to, the applicable legal rules.
Whatever your theory, non-lawyers are free to give legal, medical, etc. advice as long as they don't present themselves as qualified professionals or, in many cases, charge for it.
Professionals in the field are restricted in the advice they give.
People give legal advice on HN all the time, for example; lawyers on HN have to say 'but none of this should be construed as legal advice', etc.
Non-lawyers can't charge for legal advice, but they love to give it!