The author is drawing unwarranted conclusions. The author seems to think if the Saxons let the native Celts live on in numbers that the invasion would have been "peaceful." I suppose in this kum-by-ya age academics think it would be reasonable for the Celts without a fight just to invite the Saxons to take their best land, take over the government and make themselves wealthy by the standards of the day.
The Saxon burials that have been found have been sumptious, and the obvious conclusion to draw is the wealth interred with the dead came from plunder. Like the Vikings not long after them, the Saxons sacked monasteries. It’s ironic that the British Isles turned to Christianity first in areas never ruled by the Romans; that said, there were also Christians in the Roman areas, before and after the Roman withdrawal, but they for the most part got hacked to pieces by the Saxon invaders.