Without doing any research into this, I am guessing that it is straightforward to mosaic. Doing so gives the user more resolution to do whatever he or she sees fit in advanced or conventional ways.
Why nona instead of tetra is easy to explain as well. The smallest radius for a normal filtering kernel is r=1, which gives you 3x3. I find a 3x3 is faster and more natural to use in my own G’MIC processing than an even number one or larger odd one.