You're saying that robot will ignore auction entirely only if:
1) it has >=2 plants capacity;
2) capacity of plants on current market is lower than the lowest of the robot's;
3) nominal value of plants on current market is lower than the lowest of the robot's;
otherwise the robot will bid according to tile rule, am I right?
I'm saying that the interpretation "robot ignores the auction until coal plant appears" seems less likely to me because it could cause a robot to illegally not put a plant up for auction in the first turn of the game. (In the first game turn, everyone must buy a plant.)
(Admittedly such situations maybe unlikely, but nonetheless possible.)
So I think "it will choose between oil and uranium" in case there is no coal plant up for auction, even if coal is cheaper than the other resources.