I think that detail about GPL could be easily missed because it's written in small print, but that's a different problem. There are a lot of Qt components so it doesn't make sense to list them all on that page. A full list that can be filtered by LGPL/GPL status is here: https://www.qt.io/features
I'm assuming by outside knowledge you mean the fine print of the license: how is any company supposed to prevent your boss from having to go over that with a lawyer? This isn't even an open source thing, it applies to anyone in the software business. And this is ignoring that the GPL is probably one of the better understood open source licenses at the moment.
The key piece of information is absent.
> it seems to do as good a job as a short marketing page probably could
It could have done better in a single short sentence by stating the key piece of information, rather than leaving it to be inferred laboriously from a pile of details.
> It's not a place where they can reasonably address every common misconception
No, but it could have addressed the single largest misconception. It chose not to.
Making this inference requires you to not only have outside knowledge of open source licenses and the Qt licensing situation, but to be rather confident in said outside knowledge. That's what he had trouble with.