Simple way to think about it:
imagine that 9k who always played on 9x9 only, first time plays 19x19
his power possibly will be 15k
And what if there is 9D 19x19 player who can tell each time in which corner to play?
9k would play definitely better than 15k with that cheat.
We can train AI on 10x10 board with special border properties
then we can train AI that only able to choose 1 of 4 possible options,
small AI plays locally automatically, big AI don’t know about existence of small AI, small AI is part of rules of game that big AI plays.