And if it was a missile, traveling at, say a modest Mach 2.5, then there is no way it would have remained in view for 8 minutes. No way. And, Cruise missiles don't have a plume trail and while ICBMs launch with a plume, they fly a boost phase only as they rise up to enter a ballistic phase (with no rocket motor exhaust). They do not fly a flat trajectory.
This is all part of it (the conspiracy) man. The North Koreans have successfully launched an invisible, slow-motion ICBM from a sub that they don't have. That's the conspiracy. Get with the program.
Invisible, slow-motion, silent, stealthy, ICMB. The implications are frightening.