"Profession" and "professional" have precise legal meanings, which roughly equate to "the government gave us a monopoly on gatekeeping who is allowed to advertise themselves as one of us." That's the true meaning of those words. They've colloquially drifted over the years (through the use of phrases like "professional sports") to have a second meaning "people who get paid to do a thing."

The California Board of Professional Engineers is a professional board in the traditional sense of the word. The government gives that group the authority to decide who can be a Professional Engineer, including apparently who can say where the boundaries of a plot of land are.

There's a collision of definitions here. Since the whole case is on whether or not the professionals (in the first sense) can keep this guy from practicing, it's ill-advised for him to use the term "professional" in the second sense.

You can try to start the California Board of Jean Wearers if you want to. IDK if there are restrictions on who can call themselves a "board," but w/e. You certainly won't have a .ca.gov website (like the Board of Professional Engineers has), and you most certainly won't receive a monopoly from the government to define who can call themselves a jeans wearer (which the Board of Professional Engineers has).