The problem is NSA really only knows their motives.
The other problem with the NSA is that they have two goals. One is to protect the US and US entities security and the other is subvert foreign entities. If they start publicy recommending coreboot for what they deem senstive installations then their intentions are obviously not malicous. Problem is the NSA may never publicy specify that its for senstive installtions and may only ever be an internal guideline.