It's the old "reasonable man" test that the courts use. A reasonable man knows that guns are dangerous.
On the other hand, a reasonable man may not realize that swallowing magnets are far more dangerous than swallowing similar objects that aren't magnetic.
From the comments I've seen in other discussions about this, people are entirely capable of reading all the warning labels and still failing to realise that swallowing these is actually far more dangerous than swallowing other metal objects.
They just seem to see the warning labels as an example of "health and safety gone mad" so to speak, of someone putting unnecessary warnings on about something that obviously can't be that dangerous. I can't see a good way of avoiding this problem.
Edit: Apparently there were similar comments in the previous HN discussion; at least one person[1] failed to grasp how much more dangerous they were if swallowed than other objects despite reading a 14-page complaint that described in graphic detail the exact mechanism by which they were more dangerous, the serious medical consequences, the long-term health risks even with prompt and successful surgery, and the history of kids being injured in this fashion by them. They're basically impossible to sell safely.
This not only seems true, but demonstrably is true. CPSC worked with the vendor of this toy for years getting labels and notifications out; the reports of children requiring debilitating surgical procedures to remove tiny little rare earth magnets from their GI tracts increased over time.
Given that many people initially thought this ban was silly until they researched the actual consequences of swallowing the balls, reality does not seem to bear out your hypothesis.
On the other hand, a reasonable man may not realize that swallowing magnets are far more dangerous than swallowing similar objects that aren't magnetic.