No, you're absolutely right. The author complains that with physical possession of the device that it's possible to transfer Signal's private key material to a new device, leaving the old safety number intact.
The author apparently expects the safety number to change in order to alert the person on the other end that there "might be a hostage situation," evidently not realizing that the attacker could just, well, use the unlocked phone right in front of them.
The author apparently expects the safety number to change in order to alert the person on the other end that there "might be a hostage situation," evidently not realizing that the attacker could just, well, use the unlocked phone right in front of them.