Sky fortress is likely the EMP field they've created over North Korea with the space based emp that was put into place by the Zuma mission. This enables our friendly forces that they can be confident that the enemy they are fighting has nearly no electronic capability throughout its armed forces. This is the time that friendly forces would be launching coordinated attacks against the NK and from the sound of things they are at the very least announcing that the space based emp field has been put into place over the country enabling the possibility for much safer operations and most importantly somehow averts the instant destruction of Seoul that so many have been concerned would coincide with any attack on the North.