FAir points, but lets remember; they have super powers as well. And the armor is more to protect them when they do get the chance to get bigger. The tyrannopedes would likely go for the lighter parts, which are less protected from his armor. So if it gets a good bite at his neck, that's a game over.
Basically speaking, the armor only protects the parts of the body htat are in the darker hsade of brown area, but the area that's lighter brown (neck to torso) are less protected except by thick muscle, which likely won't help if they get tangled up in the webs.