@PhSt My thinking, its a gratitude donation, Ukraine had help the Chinese military a lot from marine engine, missile technology, radars, large airplanes ( Antonov involvement in the Y20 development) and maybe some of Motor Sich blueprints tech for engine. $3billion of lost investment is cheap compare to what China had gained.
Ukraine purchased these doses. Its a commercial transaction, not a donation. Its also helpful to expand sales in Europe, as it is seen as more credible than just Africa/ME/LATAM. Part of the long game and its worth it.