The aircraft belonged to the military but was not a transport plane; rather, it was an official business jet under the Pentagon's purview, thus qualifying it as a military service aircraft.
It had a flight number and a mission code, so Gao Guang's plane represented the United States from any perspective.
Strictly speaking, Gao Guang's treatment had been upgraded, transitioning from hitching a ride on a military plane to having a business jet specifically assigned for him, elevating his status significantly.
In conflicts amongst the gods, mere mortals suffer. Poland likely knew there was some internal conflict on the American side. This so-called military observation group was merely a pretense. A high reception standard could offend, and a low standard could offend just as much.
Thus, when Gao Guang and his team landed, the Polish military sent a colonel to the airport to welcome them, and the Ministry of Defense also assigned a civilian official to meet Gao Guang.