Exactly I also find it slightly silly to immediately declare this 'insecure' in this case here.
If it was directly end-to-end on say a Nest Hub running a release version of Fuchsia then that would be a more convincing here, as that would confirm that it can be deployed and the bug can be exploited in the wild and in production and not on a newly built developer version running in an emulator.
The writeup of finding and exploiting this bug is impressive, but whether if you can use that exploit to directly attack a production version of Fuchsia on a device like the Nest Hub is another thing, which is the same way security researchers do to break live versions of other OSes like macOS, Windows, Android and Linux.