It would be relatively easy for him to find investors willing to buy OpenAI for that amount. Note that OpenAI didn't jump at the offer because they are convincing their investors they are worth far more. But that offer still creates a data point of what they are worth that is difficult to dispute while also turning down the offer (obviously if OpenAI sold Musk would win even more).This is the same trick Musk tried to pull when he was butthurt about all the short-sellers trying to make money off Tesla stock. So he made up some nonsense about taking Tesla private and claimed Saudi Arabia was going to buy out Tesla. This offer for OpenAI was about as real as his "funding secured" tweet about Tesla. It didn't work then (and cost him $20 million in fines and his CEO position at Tesla) and it won't work now.
Sorry this obviously saddens you, but yes Musk wins this round. However, it is ultimately just a headache for OpenAI not a crushing blow by any means. Just billionaires play a move of chess.