When you fly a turn and gradually increase the bank angle, it is not long before the turn needle gets to the stop.
If you continue to increase the bank angle and back pressure up to 90 deg of bank then the needle stays pinned in the max stop. But now the aircraft is only pitching.
I think this answers the question – it must measure turn rate about the earth’s vertical.
But I am sorry I do not have a reference I can quote.