He is saying "The conclusion I’d like to draw is that at least some Buddhists, by the end of the 10th century and perhaps earlier, thought of the funeral rituals practised in earlier times by Tibetan ritual specialists as a religion called Bon." . It is clear enough. In this sentence, he is saying 10th century buddhists RETROACTIVELY applied the term "Bon"
Why not try reading it?
Yes, but of course it is retroactively since the text are from the 10th century, but according to normal scientific approach that is a confirmation of the theory that there was a religion called Bon before the 10th century.
Troy existed 900 years before the Illiad and for 2600 hundred years all we knew of it was a story -- then someone tracked the story down and found a city buried where the story said it would be three thousand years later.
There certainly was a religion in Tibet we can call "Bon" prior to Buddhism. The first Tibetan king live circa 120 BCE. Prior to that there were many generations of Zhang Zhung kings. Zhang Zhung fell not because of Tibetan aggression -- the kingdom of Zhang Zhung failed because of overgrazing and climate change (for which there is both archaeological evidence and climate evidence).
Bon claims to had a transmission of Dzogchen, etc.,, independent of Shakyamuni Buddha and Garab Dorje's lineage according to their tradition coming from a teacher Shenrab Miwoche. They place the origin of Tonpa Shenrab in Western Central Asia.
This is reasonable since it is a certainty that the Zhang Zhung people migrated to the high plateau of western Tibet from somewhere else, most likely the west and they were not Tibetan, so did not come into Tibet through the low country in Kongpo like the early Tibetan tribes.
After the fall of Zhang Zhung, some Zhang Zhung people continued to migrate east into eastern Tibet and western China, settling in the border lands there. The Naxi people are among the remnants of this migration.
Some Zhang Zhung people stayed behind and moved south into the lower more fertile valleys around Kinnaur. There exists is a small population that continues to speak a descendent of Zhang Zhung language. In my opinion, because Tibetan became the dominant tongue in Western Tibet, the Zhang Zhung people lost their original grammar and thus the small Kinnauri population that speak "Zhang Zhung dialect" of Tibetan now speak a kind of colonial patois of Tibetan with many Zhang Zhung words.
The real facts are lost to history. But we can respect Yungdrung Bon as a fellow tradition that shares the ideals, goals, and essential doctrines of Buddhism. We also should respect Bon because it is important to Tibetan culture.
Did Bon borrow extensively from Buddhism? well, in my opinion, it is very likely. Does this matter? No. Does it matter if it can all be pinned down definitively? No. Do Bonpos attain liberation through Dzogchen teachings? Yes. That, in the end, is all that matters (if you are a Dzogchen practitioner).
Do people sometimes get confused about all this? Often. Is Buddhist behavior towards Bon often despicable? Yes.