I have no idea what any of that means. That's a lot of technical language I have no interest in learning..If you understand it, you can express it in laymen's terms.
Also, this is very one-sided and lacking in context. These same people have apparently said the evidence points to a natural origin. So it seems they weighed the evidence from both an engineered and a natural origin perspective.
With all of the Jon Cohen Science articles that have been posted, I assumed you were following along.
The Proximal Origin authors are primarily Kristian Andersen, Edward Holmes, Robert Garry and Andrew Rambaut. We don't hear much from Rambaut and the remaining author, Ian Lipkin, isn't talking to these guys much any more after going back to the lab leak theory and questioning the Michael Worobey papers about the wet market being the epicenter.

Marian Koopmans, Ron Fouchier and Christian Drosten were also on the initial teleconferences. They are European gain of function virologists. They received no acknowledgment in the authoring of Proximal Origin, which was rejected by Nature. A fact pointed out by an unknown individual in an email to Jon Cohen. Cohen then forwarded that email as a 'heads up' to K Andersen who forwarded it to Fauci, Holmes and Jeremy Farrar. Farrar was the director of Wellcome Trust, a British foundation involved in funding.
Re Ron Fauchier ...
"In 2011, Fouchier and Kawaoka alarmed the world by revealing they had separately modified the deadly avian H5N1 influenza virus so that it spread between ferrets. Advocates of such gain of function (GOF) studies say they can help public health experts better understand how viruses might spread and plan for pandemics. But by enabling the bird virus to more easily spread among mammals, the experiments also raised fears that the pathogen could jump to humans. And critics of the work worried that such a souped-up virus could spark a pandemic if it escaped from a lab or was intentionally released by a bioterrorist. After extensive discussion about whether the two studies should even be published (they ultimately were) and a voluntary moratorium by the two labs, the experiments resumed in 2013 under new U.S. oversight rules."
https://www.science.org/content/article/exclusive-controversial-experiments-make-bird-flu-more-risky-poised-resumeThe furin cleavage site does what is in the post. It makes the virus more infectious.
The spike protein mediates the viral entry to the cells as the 1st step in viral entry.
Inserting the furin cleavage site on the spike protein is what makes the virus so infectious.
This explains E Holmes comment that the virus characteristics were "exactly what was expected by enginering."
And Bob Garry said "the insertion was the 1st thing he would add."
And K Andersen agreed saying "for sure".
And Garry said a graduate student could do it.
The early drafts of Proximal Origin described the furin cleavage site insertion as "optimal". But some how, the congressional testimony has changed to the optimal insertion is really only a "sub-optimal" insertion even tho it's caused an estimated 7-15, or more, million deaths.
Somebody needs to question this.
One of the comments in Slack communications was to the effect that if this would be viewed as a lab leak, then that would open the door to other outbreaks as being possible lab leaks.
Would this cause K Andersen and R Garry's work in the Ebola outbreak to be revisited ?
K Andersen, on the USAID early warning prediction program ......
"

And maybe we can put an end to the rationale used to support a zoonotic origin that there have been previous zoonotic events. There have also been previous lab leaks.
This is a funding and the future of gain of function research issue.
In true scientific discussions, why would they use burner phones to discuss the 'Proximal Origin' ?