You are trying beyond all reason to misinterpret what ‘reasonable person’ means and in extension what can reaonably be called securely locked. It’s gotten pretty outlandish.
The document you linked is a very interesting read as it takes stock of currently existing law and attempts to clear up question you might have about implementation. It states as such in several places on page 1. Page 4 indeed clears up what can be considered what a ‘reasonable person’ or ‘average Joe’ has to do to properly store a weapon.
Since you also attack the legal concept of ‘reasonable person’: The reasonable person is a hypothetical individual who exercises average caution, care, and judgment in their conduct.
So unless there’s a lot of lead in the water, the average person would not leave keys to a lockbox that contains valuables or dangerous items that allows someone else, let alone an 8yo have access to it.