I mean correlation doesn't mean causation. Perhaps the very reason the bought a handgun is because they thought they had some threat or were involved in activity that would increase their likely hood of getting killed.
The absolute numbers are low (12 killings / 100,000 pop as opposed to 8 / 100,000 in the control group).
I don't think the article claimed causation but you can't determine causation from this type of study either. There's a possibility of the data being random, as well as causality going the other way, but it is hard to temporalize getting killed causing someone to start having a handgun in the home before that point. There's also a possible third unknown and causal association that is responsible for both the killing and the prior presence of the gun. That is a definite place to do research.
R-code:
sum( (rpois(10000,10) - rpois(10000,10) ) > 3)