I'm not sure "mirage" is the right term, but I agree with your general point. They're benefiting from a terrible division. I don't think they really intended to be in this position this season, but are taking advantage of it. I think they're offseason plan was to keep the valley shallow instead of bottoming out by keeping a relatively competitive team intact with the intent of getting the youth some major league experience in the process. I think they're still largely doing that, they just happen to be on top of the division in the process. Their goal was always to be much more competitive next season, and like you mention, the deadline acquisitions bear that out too.
And they did that without dealing a top 10 prospect in the process.