The idea that they are preparing for a “likely strike” is very much a speculative statement. The very same thing can happen (i.e., Cheyenne Mountain) with the military preparing for an “unlikely strike” but it still having an outside “possibility”.
They have to prepare for the “unlikely” in order to not be caught by surprise and to preserve their capability of following up. This can all be from what China is doing, what North Korea is doing, what Russia is doing ... and then ... what Iran is doing (and that being the last on the list).
It’s always been a dangerous world.