The opposite of "real-time" in this context would be "sampling". It means that the capture represents the high-resolution time history of one particular event (one explosion) instead of fast and successively offset captures from as many events.
Once someone else receives, decrypts, and reads the message, the nexus of control in the iMessage system is really lost. "Disappearance" at the appointed time would probably be by far the lowest assurance property that the system would have, and Apple may simply not want to present properties that unreliable as an attribute of the system.