Hmm, when I follow the instructions precisely on the Graphene website for installing the platform-tools, then checking Fastboot version, I get fastboot version 30.0.4-6686687

But then when I navigate to the platform-tools folder (after copying the Factory Image over) and try to run the flash-all, I get the same error: 

./flash-all.sh: 22: [: Illegal number: 1:81
fastboot too old; please download the latest version at https://developer.android.com/studio/releases/platform-tools.html