What this is saying is that invermectin was used in 19 countries from 1995 - 2015 for river blindness. They compared those 19 countries with 35 countries who didn't use invermectin for river blindness. It's not saying they are using invermectin now. The HDI was included in different factors.

Data from 19 countries that participated in the World Health Organization (WHO) sponsored African Programme for Onchocerciasis Control (APOC), from 1995 until 2015, were compared with thirty-five (Non-APOC), countries that were not included. Information was obtained from https//www.worldometers.info/coronavirus/ database. Generalized Poisson regression models were used to obtain estimates of the effect of APOC status on cumulative SARS-CoV-2 infection and mortality rates.

Human Development Index (HDI)

The HDI was created to emphasize that people and their capabilities should be the ultimate criteria for assessing the development of a country, not economic growth alone. The HDI can also be used to question national policy choices, asking how two countries with the same level of GNI per capita can end up with different human development outcomes. These contrasts can stimulate debate about government policy priorities.
The Human Development Index (HDI) is a summary measure of average achievement in key dimensions of human development: a long and healthy life, being knowledgeable and have a decent standard of living. The HDI is the geometric mean of normalized indices for each of the three dimensions.

The health dimension is assessed by life expectancy at birth, the education dimension is measured by mean of years of schooling for adults aged 25 years and more and expected years of schooling for children of school entering age. The standard of living dimension is measured by gross national income per capita. The HDI uses the logarithm of income, to reflect the diminishing importance of income with increasing GNI. The scores for the three HDI dimension indices are then aggregated into a composite index using geometric mean. Refer to Technical notes for more details.

The HDI simplifies and captures only part of what human development entails. It does not reflect on inequalities, poverty, human security, empowerment, etc. The HDRO offers the other composite indices as broader proxy on some of the key issues of human development, inequality, gender disparity and poverty.

A fuller picture of a country's level of human development requires analysis of other indicators and information presented in the statistical annex of the report.

So it's suggesting the nations who participated with river blindness treatment of invermectin had developed better as nations to deal with covid.