Of course it's diversification. Instead of being short your own illness only, you enter a swap where you receive coverage for your own illness (thus being flat

Of course it's diversification. Instead of being short your own illness only, you enter a swap where you receive coverage for your own illness (thus being flat on that clump risk), and pay a small floating leg covering (a small proportion of) everyone's illness. You have much smaller variance, but the same expected return (minus administration costs) - just like any diversification.