Or even something far less extraordinary like billionaires betting they can fly their jet from point A to point B in controlled airspace without transponders in complete radio silence without getting caught.

The whole disappearing from radar thing in this story is the only reason anyone suspects something extraordinary or military.

Everything else about it says it's somewhat benign. The observed speeds are fairly common, they describe something behaving like a normal jet, and it's traveling in airspace the military has no reason to mess with and every reason to avoid for safety reasons.

I know of a number of psychopaths who would do this kind of thing if they had the means.