THC is not cannabis. It is a single aspect of it. Which is why synthetic forms of THC have a long list of side effects that aren't always associated with whole plant treatments. Doing experiments with only THC is misleading at best and often flat out false, as the other cannabinoids as well as the terpenes present in the plant can alter the effects of THC as well as the ability of THC to properly bind to the cannabinoid cell receptors. The study your quoting did not take other lifestyle choices into consideration. Diet, exercise, careers etc etc etc. It may have also let the participants source their own cannabis, which brings on a whole other list of problems. This is unfortunately extremely common in cannabis studies, maybe false isn't the right word, but these studies are often done with a result already in mind. IE - "Cannabis lowers sperm count" Instead of "Does cannabis lower sperm count?"
Also, the original post talked about altering the DNA of sperm, not lowering the overall sperm count.
I really shouldn't discount these types of studies, and I'm certainly not trying to point out that cannabis is a miracle etc etc. But the vast majority of the studies done on it are trying to find out why its bad for us, not just conducting proper research and letting the results speak for themselves. Again, nothing against the people who did this study...these types of results are legitimately the only way you can continue to get funding to study cannabis in most countries. It's a fairly broken system.. The legalization of cannabis in Canada will bring about some very accurate results for better or worse.