The ethics rules/courts tend to weigh heavily on the side of protecting the "client"
It's probably not reasonable, in most cases, to expect a forum
poster is your attorney, but there is no reason why you can't
create an attorney-client relationship on an online forum.
If someone makes a claim, the "reasonableness" test is a point that will end up in litigation. By clearly stating that you are not their lawyer, you are hoping to make it so unlikely the plaintiff will will, they won't bother to file in the first place. (Or, if they do, you can easily get the claim dismissed.)
Your example is an obvious extreme, no one has any reason to
assume you are a lawyer and the "advice" is very general. That does not mean you can't craft a post that would truly be problematic.
Oh yea... IAAL BNYL