This is polycrystalline diamond, which probably scatters phonons anyway, so it seems naively like using a single isotope wouldn't help much. But that's definitely an interesting fact and I think you're right that it probably wouldn't add much expense when the amount of material is so small.
I'm not sure that's too dominant? The thermal conductivity reported is close to the natural diamond, so increasing the conductivity of the individual microcrystals could still be significant.
