Do you have any reason to think that what you're describing is what happened here? It seems unlikely to me, as it would mean that the guy could have given out a decryption key that would exonerate him.
This case has very little to do with the gliding scale between certainty and potentia. The legal system usually do not care about technicalities of technology but rather the interpretation of legal arguments. In here the Foregone Conclusion Doctrine is not about the existence of the data. The idea is that the government must already know a certain amount, and the dispute is over how much they know. In the case of the journal, some courts have find that the government need to know about the journal and that they contain authentic evidence in order to argue a forgone conclusion, while other courts think it is enough that the government know about the wall safe. Courts that demand that the government know about the content usually reject the case and the opposite for the later.
It is also important to understand that its not the journal itself that get subpoena. In the later case it is the wall case, with the government arguing that all locked wall cases must contain an unlocked wall case. The conclusion of the existence of an unlocked wall case is thus a forgone conclusion, with the content within being irrelevant to the argument.
The court in this case looked at this and said "The Commonwealth is seeking the password, not as an end, but as a pathway to the files being withheld". This basically mean that they don't accept the argument that the government can request the password based on the simple fact that an encrypted disk exist. Thus the focus is changed away from the container and onto the information within, and here the court do not think the government has enough information to prove a forgone conclusion.
I'm a bit confused about what you're saying here. We've already established that a court can't force you to provide the password to open a wall case. Why does it matter whether they know about the container or the contents?
It is also important to understand that its not the journal itself that get subpoena. In the later case it is the wall case, with the government arguing that all locked wall cases must contain an unlocked wall case. The conclusion of the existence of an unlocked wall case is thus a forgone conclusion, with the content within being irrelevant to the argument.
The court in this case looked at this and said "The Commonwealth is seeking the password, not as an end, but as a pathway to the files being withheld". This basically mean that they don't accept the argument that the government can request the password based on the simple fact that an encrypted disk exist. Thus the focus is changed away from the container and onto the information within, and here the court do not think the government has enough information to prove a forgone conclusion.