[tpz]

"I am amazed at how trivial this error is."

Agreed. However...

"Probably their QA got fired after this event."

I doubt it. I have seen many QA staff at a variety of companies get so used to IE's various oddities that over time they will simply stop opening some kinds of bugs against the site on IE, even perfectly legitimate bugs like that in the article. All it takes is for product management to punt mixed content warning bugs during a few releases before QA also stops reporting them. If someone does later bring one up, over time it evolves into "Oh, that's an IE thing we can't fix." These are smart people doing generally diligent work, but they do tend to get worn down around IE.

I can't say for sure why this happens but I've seen it most with IE mixed content warnings.