So digging into the double ratchet protocol, I think the device has to have both medium-term and long-term keys available to decrypt everything in the past, and an attacker would likely steal both of those and decrypt everything in the past. The protocol then would generate future keys that the attacker wouldn't have unless they continued to control the user's device. I think it does the best thing possible, but it is still quite possible to steal all the past messages even under Signal's double ratchet protocol. It makes no sense to me that an attacker would only get one key or the other -- maybe they are presuming a cryptanalysis only attack.