You need to flip your question around - the person you're replying to is not suggesting there is data that living in Boston lowers testosterone, they are sugges

You need to flip your question around - the person you're replying to is not suggesting there is data that living in Boston lowers testosterone, they are suggesting there is not enough data included in this study to extrapolate that this effect is anything other than local. For the same reason that a study conducted on Japanese men would not produce results sufficient to draw a conclusion about all men globally, you can't draw a conclusion about all men in the US from a result observed in men in Boston. Boston has a local environment, diet, culture and ethnography that could be causal in any result seen in any study. Taken at a very simple level, you couldn't run a poll in Boston alone about anything - politics, preferred beer brands, opinions about Saturday Night Live, whatever - and assume your results would be representative of the entirety of the United States. One hypothesis for the cause of the effect being described in this study is that contraceptive pill use in women is affecting hormone levels in drinking water. Can you see how the use of a particular type of contraceptive pill could change from town to town, state to state, and that the water filtration systems may differ from town to town, state to state? And that's a pretty simple environmental factor you'd need to control for, or you could investigate locally.