I don't think the vaccine causes them to shed. The statement about them shedding it is just confirming that the birds in the study were in fact infected, as intended.
Adding to this, it's my understanding that the vaccine didn't cause the birds to become infected, but that the researchers intentionally infected them in order to test the results of the vaccine. So giving the bird the vaccine protected them (and did not make them infectious) only if they had not already been infected by ABV.
Unfortunately, since a lot of birds already have the virus and are simply asymptomatic, it may not help current populations. However, if it becomes routine for younger birds to be vaccinated, hopefully we'll see a reduction of PDD in the future.