“By default, Apple devices expose the first name of a user in their local hostnames, which we are going to use for the name brute-forcing technique.”
Apple names your Mac after you and broadcasts it to the whole local network. A web page can time fetch requests to guesses like johns-macbook-pro.local and learn your first name with no permission prompt. A list of just 50 popular names gets it right 65% of the time. The warning comes from a fingerprinting company, which tells you who has been studying this.