I can confirm this is the way it works. you 3 star the map, the the map then adds the plat unit as a enemy. if you defeat that enemy you have a chance of it dropping it self. Or at least how the 1st one was. I got the Belinda to drop same round i got black sprite. but still did not get Silver witch until 3 more runs after. Then making it complete and getting gold unit. So you do not need to get the complete to get platinum to drop, just need to 3 star the map and start farming.