I've been reading some papers about looking for neutrinoless double beta decay. A couple of them talk about finding a lower limit for neutrinoless double beta decay. From what I understand double beta decay is already a very rare process that has an extremely long half-life, but I'm not entirely sure what this has to do with finding $0\nu \beta \beta$-decay. A specific example is from this paper: Search for Majorana Neutrinos Near the Inverted Mass Hierarchy Region with KamLAND-Zen. In the paper they say they found a lower limit for the $0\nu \beta \beta$-decay to be $T^{0\nu}_{1/2} \gt 1.07 \times10^{26}$ yr. I'm not sure what this means in terms of looking for $0\nu \beta \beta$-decay.
How does knowing this information benefit researchers when looking for $0\nu \beta \beta$-decay?
Is it because knowing the half-life provides a better approximation for the statistics of finding $0\nu \beta \beta$-decay?
Also, how is it possible for them to find the half-life of $0\nu \beta \beta$-decay if it's never been observed?