It also used to be very Linux/Gtk-centric too, and part of me is sad that the focus is now on enabling proprietary platforms at the same level of robustness. But that's obviously a philosophy thing and I'm not the developer.
And to get to the point of the blog post: it's absolutely true that cross-platform sanity has never been part of Gtk+'s design goals. It's always (well, since Gnome 1.0) been a playground for stuff in Gnome, and work on other environments a secondary thing done mostly by volunteers.
From the perspective of an app developer, Qt is much (much!) cleaner in this regard. Though it gets that at the cost of staggering code size and build complexity. Really Qt has to duplicate the superset of features provided by all the supported environments. It does it, but I can't say it's pretty.