Langlais said this was so that there is a proper balance between the 'juicy' flavors and zesty ones in Cointreau. If they left everything in, the liqueur would be overwhelmingly zesty."
This can be bled off from the base of the carter head and used to make coloured spirits.
They steep the peels in the wash which is a slightly different process.
You would remove the thumper from your carterhead in order to minimise the zest.
It balances nicely with the stevia sugar mix when you proof.