I think the authors made a distinction between what they presented as historical fact and what they presented as a possibility to be on watch for. I agree though that they reported assertions by protesters about whether the chicken came first or the egg that cannot, at this time and likely ever, be either proven or disproved but they were identified as claims by protesters, not as established facts.  

In protests across the country over the past week, the clear actor escalating the violence generally hasn’t been a protester or even a right-wing infiltrator, but the police themselves. In rally after rally, people have observed that looting and destruction only began after police charged and beat a crowd, or fired tear gas or rubber bullets into it.

The last sentence in that quote seems counter-intuitive as to how the looting started. The looting started, IMO, the instant the looters became convinced they could get away with it. That said, there is plenty of evidence of uncalled for brutality by the police documented by legitimate journalists, some of whom became victims themselves. 

That brings us to the present day. On the one hand, this history doesn’t mean that the FBI or local police are currently acting as provocateurs during the current unrest. But it does mean that such activity is clearly one avenue that is open to U.S. police forces looking to undermine protests and escalate violence.