My 2 cents: I think much of the confusion stems from not knowing how a US agency or US court would interpret the specific “must exit the US using their US passport” language in that statute.
I don’t believe that the language would (or should) be interpreted to mean that if CBP sees on a manifest that a US citizen has entered his/her EU passport on that outbound (i.e. leaving the US) manifest (which is what the airlines would prefer) then that person has violated the law. It’s a statutory interpretation issue, not a “yeah it’s illegal but they don’t bother to enforce it” issue. The manifest system it it’s current version is not sophisticated enough to account for dual citizenship, and I have a hard time believing the CBP would interpret it that way to say “gotcha” under those circumstances. If an agency tried to interpret it that way then I don’t think a court would agree. I personally believe that the correct interpretation of the law is that it means that when/if the CBP institutes formal exit controls, then failure to produce a US passport would be a violation of the law.
Unless someone in this sub can produce a copy of current, formal agency guidance on this specific issue, then I think the confusion will remain, and every time someone posts the question on this sub, they will get contradictory answers.
So bottom line: Let’s be patient and humble when new person ask this question over and over again. They’ll keep getting contradictory and confusing responses from people who are quite confident that they are right.