If I'm understanding the summary correctly, the purpose isn't to have a way for the drones to defend themselves, but to have drones that can defend a Navy ship, an army base, etc.
TFA is about drone self-defence..
If a Predator drone were to get shot down [...] the bad side is that you just lost a $4 million piece of equipment. So, in a bid to keep drones protected, DARPA is funding research into drone-mounted laser weapons.
The project, called Endurance, is [...] being tasked with the development of "technology for pod-mounted lasers to protect a variety of airborne platforms from emerging and legacy EO/IR guided surface-to-air missiles."
Moreover, ships and bases already have great anti-missile defence technology - and the only advantage that would have using drones in a defensive role would be if there is poor LOS, in which case the strategists would be out of a job, if not court-martialed. Moreover, the ship/base airspace would be cluttered. Most UAV designs are for long endurance missions. the article refers to MALE UAVS (Predator / Reaper), and hints at HALE UAVs such as the RQ-4 Global Hawk and the RQ-170 Sentinel .
Note that the Iranians downed an ultra-secret RQ-170 Sentinel using EW (electronic warfare), not missiles. Lasers won't be much help with emerging EW technology.