In reading the USGA statement on Bryson DeChambeau's use of a compass, it's easy to see how they determined it to not be a "usual piece of equipment," just as the Rules forbid. (Rex Hoggard had the statement here first, and here is Golfweek's Kevin Casey with a roundup of the back and forth over Bryson and his compass, including the full statement and rule reading.
The compass and protractor work Bryson was doing certainly could be seen as fitting this description:
a. That might assist him in making a stroke or in his play; or
b. For the purpose of gauging or measuring distance or conditions that might affect his play;
One reason the compass and protractor yardage checking might not be a usual device? Most golfers, caddies and others would not know what to do with them!
Also fitting the Rules description for unusual devices assisting play would be yardage books with gradients shot by rangefinders that disallowed in competition, and of course, green reading books which are now a usual piece of equipment because they were not immediately deemed unusual soon enough.
The same green reading books where he was using his protractor to double check a hole location!
Do these inconsistencies undermine the credibility of golf's Rules? How can they not?