Your Honour, with all due respect to the Court I fail to understand...
... how disclosure "would compromise law enforcement efforts in many, if not all, future wiretap investigations." The information already available is more than enough for any interested party, be it someone who is up to no good or someone who is simply concerned for his/her privacy, to decide whether or not Facebook Messenger or calls can be considered private.
Clearly, FB can decrypt the calls in principle, at least by changing some internal working of their software. Clearly, the government wanted them to, and they refused so far. While I would indeed be interested in the arguments provided for and against, and in the technical details of the possible decryption mechanism, this is in the realm of intellectual curiousity and, possibly, civil interest (what is the government up to?).
Operationally, however, those details do not change a thing. If such decryption is implemented some woefully uninformed bad guys will be caught and the well informed ones will not, regardless of what the details are.
Frankly, at this point I see only two reason to keep the details secret. One is to make it harder for less-than-competent lawyers to come up with arguments why evidence gathered via such wiretapping may be inadmissible in court. The other one may apply in case where the secret arguments demonstrate the decryption is very, very hard indeed: there may be this idea that scaring a few bad guys and, crucuially, a whole lot of law-abiding citizens off a reasonably secure channel of communication is a worthy goal. IANAL, but I seriously doubt either line of reasoning can lead to good laws, and IMHO anyone who adheres to such arguments in setting legal precedents is not fit to be a judge.