How much time do you need? How much do you have?
They're saying that 5 million years is too narrow to show much evidence of evolution within bird lineages -- uh, how much time do they postulate for hominid evolution? Less than 5 million I think. How's that work?
Adaptation is driven by changing environments.
A few hundred thousand years of rapid frequent climate change - ice ages, thaws, warm inter-glacials and more ice ages - drives rapid adaptation (or extinction for critters that can’t adapt fast enough).
Five million years of a relatively steady warm wet climate doesn’t put much additional evolutionary pressure on critters that successfully adapted in the first few tens of millennia.
The differences in adaptation rates are exactly what the underlying theory would predict.