I'd argue that if your boss is refusing to act on a serious security concern you have a moral duty to take it further up the hierarchy yourself, and to (responsibly) disclose to the government/public if the company won't listen. Shrugging and saying "Well, I tried!" isn't good enough.
There are more things and nuances specified, but the idea of "this is critical, but oh well my manager ignored me... nothing more I can do." doesn't seem to breed organizational success. (By no means I am advocating for insubordination per se.)
I would never go public without going to a lawyer first, and getting enough proofs (emails) that I did everything I could to raise alert in a responsible way inside the company first.
Edit: after rereading the article I thing we just don't have enough data to know how things happened exactly.
> 1. Engineers shall hold paramount the safety, health, and welfare of the public.
> a. If engineers’ judgment is overruled under circumstances that endanger life or property, they shall notify their employer or client and such other authority as may be appropriate.
This first part of this is correct (first try through your reporting structure).
The second part is incorrect - it is precisely unprofessional to decide there is nothing more that can be done simply because your manager didn't act, which is what it sounds like you are suggesting.
Details always matter of course, but "well, I tried" is rarely going to be good enough.
Edit: I see the title indicates the person was a "senior" employee but I don't know what that means, since I can't read the article. Perhaps he/she was more than an IC.
On the other hand, often there are lots of options between "do nothing" and "blow up my career".