In context, how is that wrong?
Do you know if they had other evidence that put that 'plausible' higher up on the credibility scale?
See, what you fail to undertsand is that all of the information the Administration had creates a context -- a framework for understanding that information. Any single piece of information, alone, means nothing, but taken as a whole, it then has meaning.
Take, for instace, the supposed lauch site.
If there's no other evidence - like, say, a known project for the components necessary for such a missle to be built -- then the site itself means nothing. But, with that other evidence, it means more - if ther is such a missle, then it much launch from somewhere; a new missle needs a new launch site; a new launch site combined with a new missile leands to the strong possibility that the new site is for that new missile.
You ASSUME there was no other information that supported the idea that the site was for a new missile; that assumption is unsupportable.
An unsupportable assertion.