Measuring people developmentally is not an intrinsic part of Holacracy. You are picking an example of an organization that decided that this kind of measurement would be useful for them. It does not represent the practice in all companies that work with Holacracy.
Apart from that, Robert Kegan is a professor at Harvard University. His work is academically sound — regardless of how people use it, with or without the necessary skill. Developmental assessments are tricky and I get why their unskillful use is highly problematic.
But again, this is NOT a part of Holacracy.