I guarantee you that some of the fighters who have a TUE have them because they abused PEDs. Low testosterone for someone under 40 is relatively rare, and even then it's usually caused by obesity or diabetes, which we don't see often in MMA. Furthermore, there is no way that these men got to the highest level of their sports having naturally low testosterone.
Frankly, I don't mind TRT in those who have low test for whatever reason, as long as it gets them to normal levels as the average fighter. It provides for better fights, and has health benefits for the respective fighters.