The thing is obviously a fake/decoy.
If they had a functional "nuke" and wished to spare peoples' lives, they could easily have re-arranged some innocent countryside within easy view of the enemy to make their point.
The fact they didn't, tells us whatever's looking mighty impressive up there is either non-functional, or so ridiculously expensive/impractical to fire off it's basically useless.
There was a quite valid point to Nagasaki, after all.... It told the Japanese government very unsubtly Hiroshima was very definitely not a one-off bluff...
And besides his inferiority complex, Brother-dear is not a slouch as a tactician. If the thing was practical twice he would never have hesitated to use it.