Changing the behaviour of a file mode from truncate to not-truncate is a questionable decision because these are very explicit options a developer would carefully select, and therefore would not expect to see a change in behaviour to something already covered by a different option.

I work in finance and I can confidently say it would be at the top of my mind that this kind of change would result in leaking data or corrupting data because I regularly explicitly choose truncation to avoid exactly that when I generate new reports.

It's also ironic that you complain about hyperbole then accuse people of "pretending to be smart". You don't need to be smart to see this as a security issue, you just need some real world development experience.

My takeaway would be that they should have security-brained people screening "non-security" bugs, to check for potential security relevance.