Drawing a "line in the sand" this early seems rather unwise.
Yes, there needs to be some method for determining if a question is, or is not, on topic for the site. Every stack has to settle that point. Although that point is often in flux on established sites as well. While in beta, especially early private beta, there will be many questions that seem on topic, which later are found to not fit so well after all. Likewise, some questions that are off topic now will probably turn out to be quite on topic at a latter date.
I think it's going to be better for the site's development if borderline questions are handled individually, maybe even after some discussion in chat or on meta, rather than trying to create a set of rules or drawing lines while site scope is still very much open for discussion.
As to the instant question, it seems uncertain why the OP needed to "explain" things to the layman. It seems that it is purely quantum mechanics, and no quantum computing connection at all. As such, I'd think that question is off topic. However, if the OP was trying to explain QC to and the QM was the sticking point in that process. Then, maybe, it would be on topic after all. Then again, maybe not even then.
As to trying to limit on topic to "questions that are about the effects of QP as they relate to implementations of QC", seems rather limiting, either to topicality or to question content. I can conceive of users working with QC having problems with something and posting a question here. Being used to general SE guidelines, they limit the info in the question to what's needed, without a long backstory, thereby excluding that the problem relates to what they're trying to do in QC. Other users see the question, don't see any mention of QC and close as off topic, even though with the mention of the QC connection it would have been considered on topic.
I think that with a question being posted on QC.SE, I'd be inclined to presume there is a QC connection, until shown otherwise. (The instant question seems to not be connected to QC, and does need to be clarified if there is a QC connection to it.) Your suggestions seems to be that there should be a presumption there is not a QC connection unless stated otherwise.
As a final suggestion, it might be possible for a question that's borderline to be edited such that it becomes obvious that there's a QC connection, and what that is. By preference, such ought to be done by the OP, but other users can attempt edits as well, keeping in mind the normal guidelines for non-OP edits. Thereby some questions might be salvaged as well.