I'm sorry that you have run into this and the proverbial dead horse has been well beaten with pointing out the lack of QT..
I'm no vet but I have experience with RI's and I can tell you I've had snakes develop RI's due to the stress of shipping and being in a new home in a matter of just a few week's. So in that fact your vet is incorrect I've seen it in my collection first hand..
Also people tend to focus on keeping their existing animals away from the new ones when the QT, but its two fold..there is the possibility that the new snakes might have caught something from your snakes and not the other way around..
Thanks for the info..