This seems insignificant when compared with other selector genes such as H63D and C282Y, which could potentially double or halve subjects life span depending on environmental factors.
However, I find it very interesting that all those important details seem somewhere hidden in the paper. Also I guess if you consider a 10x increase between the species and then end up with 4% this sound really far less spectacular.
A fairly average lifespan is 80 years, so a 4% increase would be about 3.2 years. If I were offered a "wonder drug" that would give me 1,168 more days with a loved one, I would take it.
Obviously that is not how science like this works, but stacking small victories over time is a way to achieve "significant" impact.
But what stigma and complications/risks could they live with for a 4% increase in lifespan? What if they find out in 30 years time that their GM means that they can't have kids with their partner or something because there is some biological incompatibility that wasn't anticipated and it just miscarriages. What if some country is Xenophobic to the idea and won't grant them residency or a passport because they are GM, or they are eventually targeted by some kind of anti-GM hate group? Would that 4% be worth it? Like everything it's a risk-reward value prop.
A nitpick: Those are not genes, but mutations, probably describing variants of the HFE gene (according to a quick search).
It's very similar to the well established fact that students with good grades are the most likely to cheat.
If you want to look for solid research, don't look to Nature, look to the so called "mid tier" journals. Also, similar observations are true for research groups. Research from ivy-league universities follows similar principles as above.