Chauncy said in "Fool Moon" that according to legend, St. Patrick cursed the MacFinn bloodline to become loup-garou, passing from generation to generation, and that the cursed bloodline would last until the end of days.
In light of this, I see four possibilities:
1. Chauncy was lying.
2. The legend was inaccurate.
3. The end of days had already arrived as of the end of "Fool Moon", ending the curse on the MacFinn bloodline.
4. Harley MacFinn's death did not make his bloodline extinct; he had at least one younger relative, who inherited the curse.
Item 1 is almost surely true, though it may not be the complete explanation.
If you go back and reread that scene, there's an interesting point. Up until the curse came up, Chaunzogorroth had answered all Harry's questions straightforwardly, with simple declarative statements. Yes he is this, no he is not that, etc.
But when the curse was the subject, Chaunzogorroth suddenly switches to 'It is said..."
That way, even if what he's telling Harry is partly false, completely false, or a mix of truth and falsehood, or whatever, he still isn't technically lying as long as somebody somewhere
said it.
Now, Chaunzogorroth is not a Sidhe, he can certainly lie, but it he wants to be summoned to Earth by lazy naïve/arrogant suckers like Harry, he has to be offering something at least potentially useful to bait the trap. So he can't afford to get a rep for lying. It would remove the only reason to summon him in the first place.
Which suggests to me that he was almost surely
lying about at least some aspects of the link between Patrick and MacFinn, but lying in a Mab-ish way with the 'It is said'. After all, if matters were just as he was saying, he wouldn't need to say, "It is said...", he could say, "Saint Patrick cursed the family." But he did not.