One thing to keep in mind is that as transmissibility increases, lethality decreases, and that’s not coincidental. The longer it takes for a virus to act, the longer a host body has to suppress it and adapt to it. Fast-acting and highly-lethal viruses depend on carrier populations (like pangolins in this case) to survive, and many individual viral mutations with deadlier characteristics emerge and then die without ever even reaching another host.
Is it possible that you could genetically-engineer a virus to both have a long incubation period AND incredibly high mortality rate? I’m sure it is, but that is not the claim here, just that they are experimenting with a high-mortality, naturally-occurring Coronavirus.