I would be shocked if you could find a filesystem developer who isn't aware of exactly what POSIX requires of rename (i.e. atomicity with respect to other processes viewing the same fs concurrently), and why it works the way it does.
Our standards keep rising, though, and these days people actually care about what happens to data over crashes -- and POSIX's primitives *suck* for this, unless you are writing a giant database with dedicated storage.
In this discussion, whenever kernel folks talk about developers coming to depend on rename's atomicity, I'm pretty sure they're talking about its atomicity with respect to crashes. (For instance, I believe Subversion's backend format uses atomic-rename for reliability over crashes, because fsync is just untenable.)