If it’s local to one part of Nigeria, there could be other environmental factors there. The study has no sperm samples from before the ivermectin, nor a control group of participants with similar demographics who didn’t take ivermectin to compare to.
They had a built-in control, but the people using this don't need the study to have been done scientifically.
In this study we screened a total of 385 patients who were diagnosed of onchocerciasis. Out of which, 37 (9.6%) were eligible for further tests, as their sperm counts were normal while the remaining patients had very low sperm counts and were therefore not used for further tests or were too weak after the preliminary screening tests and were not considered eligible for further test/studies.