I think you missed the last sentence of the post:

Finally, when you reference a Git hash for posterity, e.g. in another commit message, I’d recommend always using the full value.

The git config is just for display purposes in terminal output. That only needs to be unique as of the time it’s displayed; and as I noted, the current default behavior is to adjust the size dynamically, so the displayed hash segment is always unique no matter how big the repo is.