How can microinverters be as efficient, or more, than power optimizers of solar arrays?
With something like solar power generation efficiency is a tricky thing to pin down. 

The micro-invertor itself may indeed be more efficient than the power optimizer, but it does that at a cost of much higher currents in the system. The latter translates into much more expensive wiring, connections, switching systems, and ultimately the micro-invertor itself. All of those can also add significant resistive losses in the system if you do not spend enough money on them that take away from the efficiency before you even reach the invertor.

Further, to complicate matters, when considering the efficiency of solar systems you also need to factor in costs, specifically costs per kW over the lifetime of the system. A system that produces power at less dollars per kW can be considered as a more efficient system even if it is extracting less raw power from the panels. This is especially true if you are trying to redeem your investment costs by feeding back into the grid at a fixed price.