BB may be technically birdshot, but it's not what anyone who hunts birds means when they say, "birdshot".
When I say, "Birdshot is totally inappropriate for self-defense" I'm referring to actual
birdshot, used for bird hunting from pheasants on down, which is lead shot from size 4 to 9.
What you want to call BB, T, or whatever is up to you. I wouldn't call it "birdshot" or "buckshot". It's typically used for turkeys (class
Aves but a very different style of hunting from "bird hunting"), coyotes and other farm/ranch varmints, prison rioters, etc. It's often found under "specialty loads" in a catalog or store.
I'm sure BB works fine for self-defense. Lead BB, anyway, not steel. That's not what I mean, or anyone else who hunts birds means, by "birdshot".