I really liked their take on the countermeasures: "It is possible to reduce the amount of light reaching the microphone’s diaphragm using a barrier that physically blocks straight light beams [...] However, we note that such physical barriers are only effective to a certain point, as an attacker can always increase the laser power in an attempt to compensate for the cover-induced attenuation. Finally, in case such compensation is not possible, the attacker can always use the laser to burn through barriers, creating his own light path."

Sure, you can try to defend by putting a lid on the microphone. But we already have a laser here, we'll just burn through that.