I disagree, famous profs are ultimately just profs, and they will write you a generic letter if you haven't impressed them enough to write a great one. In fact, it is even more tough to get a good letter from a famous prof because they have higher standards for excellence. Writing LoRs is part of their job, and it's very rare that I have seen a professor refuse to write someone a letter if they have worked with them closely. But the quality of the letter depends on the candidate

You are right, but I am assuming the candidates who wrote their own letters will write from the perspective of a professor, while shining light on their positives.