This study only makes sense if most of the individuals within the study were primary hypogonadic. If secondary, hCG would've increased their test levels a lot because it means their testes are functioning normally but the issue with their low t lies elsewhere (usually with the signal from brain to testes).
If someone has primary hypogonadism, then their testes aren't producing enough testosterone even with a healthy signal telling them to, so added hCG won't work well in this case.
This study above is useless if they didn't take any of this into account during it.