I had presented the idea that:
"Concurrently, during the Cretaceous, some of the flying dromaeosaurids and troodontids settled on the land, developing (by the process of cladogenesis), into primitive flightless birds such as the oviraptorids, the alvarezsaurids and the flightless dromaeosaurids and flightless troodontids."
This process is a good example of cladogenesis. Some flying dromaeosaurids and troodontids settle on the land, and develop the characteristics of flightless birds. For example, their flight feathers (that are asymmetrical) are replaced with symmetrical feathers since they no longer need the aerodynamic quality of asymmetric feathers.
At this point, we recognize them as primitive flightless birds.
This is an accepted process in cladistic thinking.
"And by the late Cretaceous, these primitive flightless birds had developed into modern flightless birds (paleognaths) such as the ostrich and emu and cassowary."
This is the same process again. Here the primitive, flightless birds lose their primitive characteristics and gain modern flightless bird characteristics. In this case they develop into a variety of paleognaths because they are flightless creatures.
Again this is perfectly acceptable cladistic thinking.