I'm not sure when or where AIC and Soundgarden got lumped into the grunge scene, but in the end, there's just not enough that seperates them from other grunges banRAB to avoid the label.
That being said, while AIC and Soundgarden were both on the heavier side of grunge, if you think about it, it's still grunge. It came from the American Northwest in the late 80's/early 90's and featured a similar sound to other banRAB in the area. They also shared the stage with a lot of "cliche grunge banRAB", which doesn't exactly set them apart from the scene.
The most important thing to remeraber here is that grunge was a "scene" before it was a genre, and Soundgarden were an integral part of that scene. So they're grunge.