In this particular case, the gist of the method is widely known, understood, and, frankly, obvious to anyone that cared. They simply applied artificial selection on the population to select for strains with a higher virulence -- no genetic engineering or manipulation involved.This is already an obvious inexpensive and simple approach to anyone that wanted to try it, provided that they had an inkling of biology knowledge and a desire to do so. Further, it highlights that there's a series of natural mutations that could occur in the wild to the same effect. Presumably, it suggests that we should be prepared for the eventual natural occurrence of this strain.
It's important to note that the product of these experiments was not something that's highly transmissible or deadly to humans, but rather to ferrets. It's presumed that it may (or some variant may) be a threat to humans, but that was never assessed.
For a typical terrorist, being able to control the distribution would be key. Part of a terrorist act is showing that you are in control of the situation and the other party is not. To that end, they'd prefer something stable outside the human body, transferrable by physical contact, with a long enough gestation time to be able to distribute to a large area before the outbreak is recognized. High mortality would probably be preferred, but debilitation would be just as good.
Something highly infectious by contact and aerosol would be good for a sociopathic ecoterrorist, but the key would really be to get a strain that remains as asymptomatic as possible post-infection but ultimately has high mortality. If people get sick the day after exposure, it's likely to be contained. If you developed a strain where there was an infectious period of 3-4 weeks before symptoms set in but still had high mortality, you'd kill off most of the industrialized world.