OK, but he said "entirely subjective", which I interpret to mean "entirely based on or influenced by personal feelings, tastes, or opinions". Richard is right if you use "entirely influenced by...", but I disagree with "entirely based on..."
(And Rich, I agree that course slope is a close approximation of difficulty. However, if each player in your rater's group is a 5 or less handicap, then the overall course rating accounts for a significant component of course difficulty. If a course had an odd rating like 75.0/121, that's still a very hard course.)
I'm in the group that believes evaluating golf courses is more objective than subjective. However, the process of rating a course is too complicated to break down into discrete components. You play it, you look at it, you think about it, and then your mind says "7" or "6.5". I believe the experienced rater can consider everything about the course, set aside most or all of his/her prejudices and give a good answer to the question, "How good is this golf course?"
I once played a course with a friend who is a course rater. Neither of us had played the course. It was in spectacular condition. After the round, I engaged him in conversation for an hour or more about it. I like to recap, him not so much. In general, he loved it and touted its strengths. I liked it alright, but spent a fair amount of energy emphasizing its weaknesses. Eventually, I said, "OK, what are you going to rate it?" He said 6.5, and I said yeah, that's exactly what I think.
Finally, let's assume you have a data set of 500 ratings for a certain golf course. If the ratings generally fall into a bell curve with a reasonably tight standard deviation, that supports the idea that course rating is objective. If the ratings are random with a large standard deviation, that supports the concept that course rating is subjective.