‘there are limitations in the human visual system that serve to make collision avoidance difficult by visual means alone.’
Notice they say "difficult" but not "impossible"
‘small targets may be hidden behind aircraft structure, such as the engine cowlings, canopy arches, wings, or struts, until very late.’
This means you have to be "rubber necking", ie making head movements to mitigate these blank spots.
Basically I am with Captain Stable on this. Do I use radar services? Yes of course but this is additional not instead of maintaining an adequate lookout.
Lookout skills are rarely taught comprehensively other than within the military environment (and I speak as one who was civil trained but has, for a few years, instructed for the military).
When I teach E of C Part 1 and the Airmanship topics I emphasise that lookout is an acquired skill.