tl;dr probably from a sick animal, maybe from a lab, no smoking gun either way. Probably not engineered or lab-adapted.
Despite having a novel furin cleavage site in the exact location as stated in a grant proposal, a furin cleavage site that happens to make the virus extremely infectious towards humans all while leaving no trace of how it adapted this feature.
This seems to me more like a desperate attempt to avoid any regulations or attention drawn to this reckless research conducted all over the world!
In nature that happens by recombination which takes many millions of infections to emerge. In the lab you are just replicating that process by serial passage through animals where you might have a dozen animals serially infected. And the chances are probably lower because it is less likely the lab animals would wind up coinfected with another virus that would act as the source material. "Gain of function" research isn't magic.