WriteNumbers are a logical clock provided by (last I checked) a single coordinating HBase node, which is a totally valid and commonly used way to order operations. For the purposes of this article, though, I'm trying to focus on LWW data models and wall clocks. A discussion of every database's approach to ordering would take a little longer. ;-)
This presentation might provide a helpful overview: http://www.slideshare.net/cloudera/3-learning-h-base-interna...
The relevant discussion starts at Slide 16