And this is was what people warned us about when FB became the new single destination. It became its own walled off version of the web.

I think it's nice. It provides some additional accountability that Craigslist doesn't provide.

I don't know what you mean by "walled-of version of the web". At least, I don't know what kind of warning you are talking about, with respect to Facebook Marketplace.

I think they mean Facebook is the new AOL where it has it's own content (Marketplace) that non-users can't access. While Craigslist is available to anyone on the web.