TL;DR: My assessment is that a request for a simple explanation does not conflict with the site's purpose of providing "expert-level answers", so such a request does not make the question off-topic.
I posted the comment on the linked question regarding on-topic usage, so let me try a response.
Question: Are all questions asking for simple explanations really off-topic?
I think this is an important question for Meta, so thanks for asking it. It's important because it draws our attention to the core purpose of our own site, as well as the broader intention of the Stack Exchange network, the way SE operates, and how we should approach community moderation on this site.
Questions versus answers
I think there are two distinct issues involved here: the site's purpose in providing answers, and our filters as community moderators for what are acceptable questions (for which being on-topic is only one of many criteria for closure).
Stack Exchange quite deliberately emphasises the difference between questions and answers. For example, a user only gets half the points for an upvoted question as an upvoted answer; the system makes closing someone's question much easier than deleting someone's answer; and there's a penalty for downvoting answers, whereas there's no penalty for downvoting questions. Why is there this difference? The best answer is given in this blog by SE's co-founder: Optimizing For Pearls, Not Sand.
I've noticed that, at least on the five Stack Exchange sites I'm active on, a surprising number of users are unaware of Jeff Atwood's explanation regarding Stack Exchange's different approach to questions versus answers, and I've seen that this sometimes leads to confusion and errors in community moderation. For example, I've previously posted on Astronomy Meta that some users tend to be too quick to flag/vote an on-topic question for closure, when they should have either improved the question by editing it, or simply downvoted it as not useful.
What does expert-level mean?
I interpret "expert-level" to mean that the content is correct, accurate, and contains nothing that experts in that field would regard as plain wrong. If, instead, we took the Help Page's dictum for "expert-level answers" literally, it would raise the bar to the equivalent of [Physics.se]. This would exclude active contributors like me from answering any questions here, as I have no formal degree or professional experience in astronomy or related fields that would qualify me as an "expert". Yet I've contributed 24 answers here, with an average of 3 votes per answer and no answer with a negative score, which suggests that my contributions have been both useful and welcomed. I get no sense that this site really wants to bar users like me from answering questions.
Is "simple explanation" compatible with "expert-level?
I genuinely think this falls in the "interpretation and flexibility" category – in other words, it will depend very much on the nature of the thing being asked about, the degree to which a "simple explanation" can be given from an expert perspective, and the degree to which the asker is insisting on constraining potential answers in a way that doesn't benefit the site overall.
An example might help. If I ask "how is the stress-energy tensor derived?" but I add that I want the answer explained in simple high-school terms, I would regard this as on-topic but potentially hard to answer. Yet if someone could provide an answer using high-school physics, analogy to other systems or concepts, and lots of handwaving (i.e. giving an expert response made understandable), I would argue that the answer has achieved our site's purpose.
The question linked to in this Meta post asked for an answer "in simple words without using astronomy terminologies". @uhoh provided a good answer that met these requirements, and I see no conflict with the site purpose, as the author used their expert knowledge to craft an answer that was correct, accurate and useful.
However, it turns out that a similar question had already been asked, and the community of users voted that the other question provided an adequate answer.
There are three answers that were posted before the duplicate question was closed, and all three have multiple upvotes indicating users found them useful. Since duplicate questions are retained on the site, the three answers now provide an additional resource and benefit for future visitors. In my view, they are helping to achieve the site's purpose.
Of course, if the question had been closed as a duplicate before those three contributors had been able to post an answer, and someone believed the answers to the older question weren't adequate, a simple solution would be for that person to post their "simple explanation" answer at the older question, perhaps with a preface that it provides a more direct and less technical response that the other answers are lacking. This, too, would (in my view) support our site's purpose and add to the value of our Astronomy library.