Their assumption is that the outer Solar system is not particularly special, which I think is plausible in terms of primordial black holes (PBHs) anyway. If that's true then they say that a non-detection of PBHs over the lifetime of the LSST would bound the fraction of dark matter which is in PBHs of ~5 Earth masses by a few times 10^-5. They say that this is orders of magnitude better than current bounds on the DM fraction in such objects.
I'm just paraphrasing the end of the paper here, but assuming they've done their sums then yes, this is potentially significant experiment in that it will either push the bound for the dark matter fraction which is in such objects down (with assumptions that the outer Solar system is not special) or it will find one, which will be cool in other ways but will also tell us that, perhaps, some nontrivial amount of dark matter is in PBHs.