It's unclear. Depending on the fabrication constraints/robustness constraints they added to the inverse design procedure (which they don't really mention in the

It's unclear. Depending on the fabrication constraints/robustness constraints they added to the inverse design procedure (which they don't really mention in the paper), it could be either extremely robust or extremely sensitive. Generally, in terms of measurements, if the conditions are controlled, it's pretty easy to get small noise (especially in the case where these devices are so large, as is their wavelength) relative to most perturbations that happen in the lab. Overall, this is not really mentioned in the paper (unless I missed it), so I'm afraid I can't give you a direct answer on what they do, just that it is possible (via this method) to design a device which is relatively robust to these changes.