He can only back out is Twitter's securities filings are so wrong that profit projections are more than 40% off (MAE standard) or he was unable to secure financing. Spambots being a bit higher than projected is not enough to break the contract, especially given that Musk waived due diligence and that the Twitter filings have always said that the bot number is an estimate. Elon Musk's lawyers are probably more than a bit panicked right now.