I don’t know what POSH stands for (web searched to no avail), but I don’t see why it is problematic. In fact, one of the best reasons for piggybacking on block-quote syntax is that results in gracefully degrading content – a reasonable rendering by older tools unaware of the new admonition syntax.
In fact, people have been doing this for years in READMEs and other files.
You can get pedantic about whether, in these gracefully degraded renderings, an admonition is a subclass of block quote. But from a purely visual perspective, it works great. At the end of the day, human readers of content can tell the difference. As to machines, it’s a matter of teaching them, and you have to do that whether you overload block quotes or use entirely different syntax.
My beef with GitHub’s approach is that they use English.