Regardless of its uniqueness, it can't be used by that data controller to identify you because the collector does not have additional information.

I can't identify you by reading your car licence plate, even though that information can be used to identify you by others who have access to a database of cars-to-owners. The point of the law is that I can't use it to identify you, therefore I don't have to notify you when I collect that information.

Is? Or was?

Was the law written when mass tracking of inidivuals by unique serial numbers was common, possible, or feasible?

But I wouldn't be worried about just MAC tracking, you get far better and accurate information by tracking probe requests. Just by capturing probe requests you can at times guess where the target lives, works and even where they spend their free time.