The editors of the journal forced them to admit in the article that it is not clear from their data whether the higher rates of household gun ownership caused or resulted from the increased number of homicides. "It is possible, for example, that locally elevated homicide rates may have led to increased local gun acquisition," they write.
Not only possible, but likely. Which is more likely, that buying a gun causes homicide with a knife, or that high knife homocide rate causes self-defense gun purchases? Having guns would not cause stabbings. People in high-homocide areas are buying guns for self-defense.
But the Harvard group uses the study to put out press releases claiming that research supports their anti-gun message. The Reuters story said:
The study findings imply "that guns, on balance, lethally imperil rather than protect Americans," lead study author Dr. Matthew Miller of Harvard School of Public Health in Boston, Massachusetts, told Reuters Health.
Miller is an embarrassment to Harvard. He didn't even look at how guns protect Americans, or do anything to measure the "balance".