The update wasn't for your specific machine. The update does not specify exactly which GPU's need it. If you Software Update indicates your machine does not need the update then you need to trust Apple knows what they're doing and your machine does not need that update. It's really that simple. I was able to locate this CNET article, while it doesn't specifically say which GPU's but it does say Radeon. Exactly which Radeon GPU's it doesn't say nor does it infer all Radeon GPUs.