I wouldn’t put any faith in that study. Look at the range: 300 ng/dL total testosterone is what you see in 90 year old men. (Which supports the theory in some degree but you have to probe deeper and ask - Do I want to be that 90 year old skinny weak man?). It reminds me of a study I read where in a certain part of the world people who eat this plant-based diet lived to over 100 years old. Wrong metric in my book. Have you ever seen a 100 year old person? My grandmother lived to 102 and I wouldn’t want that for myself. So, the studies do not factor quality of life.
Besides, the only real metric that we know is that low testosterone has a link to mortality. So I wouldn’t waste any time trying to figure out if a particular range is going to make you live less so long as it is in a normal “healthy” range.
In the book, 'The Blue Zones' the focus was on areas of the world where men and women have extraordinary longevity. One place was the Greek island of Ikarias. They eat a mostly plant based diet, drink homemade wine, homegrown honey, dairy products from goat's milk and whole grain bread. Men in their 80's, 90's, up to 100 claimed that they were still sexually active, which could mean weekly to once or twice a month.
In the 1900's Elias Metchikoff, Ph.D, a Russian microbiologist, explored cultures that consumed fermented milk products, daily, and believed that was a key to their longevity and, to good health, in general. However, he also acknowledged that where they lived and how they lived were likely factors, also, to longevity.
For another take on this, there was an African-American women in DC, 100, who said her good health came from drinking a glass of Guinness every day. There are many interesting. even tantalizing areas to explore in regards to finding the keys to robust health and longevity, including the role of hormones, but a 'formula' has yet to emerge. And genetics/epigenetics are significant critical factors. For me, at 71, I'm still physically strong, not from exercise. Seems to be a genetic endowment. My grandfather and father, both small and wiry, had good physical strength. I'm less wiry than I was 30 years ago, but I still do push ups and use hand grips(100 repetitions). I walk a lot. Hip and ankle issues ended my jogging a few decades ago. But, severe mental illness and poor sleep have been eroding my health for 17 years.