What's the political motivation for the DNI to lie? Clapper isn't really a politician and as far as I know he's not affiliated with any party.
No one who gets any position that requires Senate confirmation is "not really a politician" (and, yes, that includes general/flag officers in the military) -- they may be purely bureaucratic rather than electoral politicians, but they are politicians nonetheless.
But, yeah, I don't see a lot of evidence that Clapper is a Democratic (or other) partisan.