I believe you've misread the license. Google is granting a license to all patents they own or will acquire in the future, required for VP8 as they released it.
If you modify your own version of VP8 so it infringes some other patent that wasn't infringed by the original, you don't automatically get a license for that other patent. In other words, you can freely make VP8 implementations without worrying about Google's VP8 patents, but you can't write your own patent license for any Google-owned patent you want by writing that patent into your own version of VP8.
What you're saying has nothing to do with what I said. I will concede, however, that I missed the bitstream license, which grants some protection to independent implementations. But as others have said, that is no guarantee.
Are you referring to this: You're covered unless Google decides they don't want to license one patent claim or another.?
Google is granting a license to all Google-owned and Google-acquired patents required by VP8. Google will be the largest user of VP8, so it is in their interest to acquire or defeat any VP8-related patents that may be discovered.
If you modify your own version of VP8 so it infringes some other patent that wasn't infringed by the original, you don't automatically get a license for that other patent. In other words, you can freely make VP8 implementations without worrying about Google's VP8 patents, but you can't write your own patent license for any Google-owned patent you want by writing that patent into your own version of VP8.