For those interested, it seems to use classical grids to align elements, and it is the only way to do it, since there is no mention on coordinates (if I read it thoroughly enough).

  void uiGridInsertAt(uiGrid *g, uiControl *c, uiControl *existing, uiAt at, int xspan, int yspan, int hexpand, uiAlign halign, int vexpand, uiAlign valign);

The overall feeling is that it’s a [small] subset of GTK. I think they try to fill an empty cross-platform niche between lightweight and native-looking Tk and heavies like GTK and Qt. Doc could explain how is that better than Tk itself, but it has to be written. The other possibility is that they do it for fun and distinctive use cases are shallow – this is often seen in second order ui kits.