Is there some obligation for a licensed engineer to disclaim non-engineering commentary?
It's been my experience that most non-lawyers on HN/Slashdot posting these disclaimers do so because someone once told them they'd be risking unauthorized practice of law charges if they didn't. That was the case once...before the first dotcom boom...Since then, NOLO and others have successfully challenged these restrictions on offering generalized legal advice in most states (including the ones that matter). Nowadays, you're only at risk of unauthorized practice of law if you're applying the law to a specific person's set of circumstances in a manner which clearly indicates that you are specifically providing advice (or other legal services) to them based on those circumstances. This is why NOLO and other guidebooks don't run afoul of these laws--they only provide general advice, it's not tailored to any particular person's legal situation. On a similar note, applying the law to a third-party's circumstances as part of a discussion is not the practice of law, it's commentary.
It's different for a lawyer. Lawyers are held to higher standards when it comes to online advice, but the risk there is whether the advisee believes that a client relationship has been created opening the lawyer up to malpractice liability. I could say that "Uber should do [X] to deal with legal problem [Y]" because it's obvious that Uber, as a well-funded company, has its own lawyers and would not treat my commentary as actionable legal advice or as resulting in a lawyer-client relationship. But I couldn't say "somethirdpartyperson should do [X] to deal with legal problem [Y]" because that person could reasonably treat commentary by a lawyer suggesting a specific action to address a legal issue as legal advice they can act on, and this belief is legally treated in most jurisdictions as creating a de facto lawyer-client relationship.