They are only 1% off. They’re trying to say “there are two independent 10% probability events that imply RTSC”. You can’t add them, but you can multiply 90% by

They are only 1% off. They’re trying to say “there are two independent 10% probability events that imply RTSC”. You can’t add them, but you can multiply 90% by 90% and get an 81% chance of not RTSC, or a 19% chance of RTSC according to this model.