As others have pointed out, this board has no skin in the game. They just voted out founders who do have skin in the game (although through roundabout means). It’s a very tough sell that this board is doing the right thing.
He convinced other members of the board that Sam was not the right person for their mission. The original statement implies that Ilya expected Greg to stay at OpenAI, but Ilya seems to have miscalculated his backing.
This appears to be a power struggle between the original nonprofit vision of Ilya, and Sam's strategy to accelerate productionization and attract more powerful actors and investors.