... I doubt that Chauncy is a congenital liar ...
He's a demon. "Lying" (like violence) is his
natural state of being.
... If he was not a true source of information, there would be no temptation to call him up by mortals putting their souls at risk ...
He is *capable* of providing true information.
But he does so tactically, strategically.
Where possible (and strategically advantageous) he misleads (recalling that overt lies from Down Under(*) are one of the things that can trigger extra Response from Up Above).
Chauncy's overarching lie is implicit: that he can be trusted, that he is a reliable informant, that the risk is worth the gain.
Note that by making an issue out of "was it or wasn't it St. Patrick," Chauncy
entirely avoids addressing who actually Cursed the MacFinn. I suspect the
real information is actually important (since Chauncy took some care to obscure it).
You recall: by the end of Chauncy's scene, Dresden was shaken, and realized:
he had been falling for just this lie.
Dresden never summons Chauncy again.
(*) Apologies to all antipodeans, who (I presume) are not all lying demons