Laymen knowledge here. My guess on the limitation is either on the targeting abilities of both nodes being able to get absolutely perfectly aligned down to a mm across stellar distances for maximum amount of light emitted by the sending laser to the optical receiving satellite OR a lens on the sending laser that spreads the laser light beam out a bit so alignment doesn't have to be as precise.

it is essential to continuously direct the laser beam, which expands only about 500 m, accurately to the counterpart satellite even at a distance of 40,000 km.

The cost to the beam spreading is that there are fewer photons per square mm to impact the optical receiver. So in this case the receiving satellite only has to be within a 500m cone to receive the signal, but thats at a distance of 40,000km. 40,000km is chump change in solar system or stellar distances, so that cone of light would be dramatically larger if the satellite was farther away, which means dramatically fewer photons to receive.