Intriguingly, That complex is specifically shielded from EMP attacks, and recently upgraded. Maybe 10 days of darkness was/is a potential EMP attack.

Per Wikipedia,

"In early 2015, Admiral William E. Gortney, commander of NORAD and NORTHCOM, announced a $700 million contract with Raytheon to move systems into the complex to shield it from electromagnetic pulse attack, with additional work to be done at Vandenberg and Offutt.

According to Adm. Gortney, "because of the very nature of the way that Cheyenne Mountain's built, it's EMP-hardened. And so, there's a lot of movement to put capability into Cheyenne Mountain and to be able to communicate in there"