Well, I don't think all the Titans were dealt with by the Olympians because several were on the Olympians side. From there, we can assume that his statement is more of a generality. We also have Gard telling us that the Fomor are a collection of defeated entities from various cultures. With all this, it wouldn't be out of place for one of the Titans to have escaped the Olympians and have found a place with the Fomor, or, depending on the time frames, for the Fomor to have found a place with that Titan.
What I found interesting was the last part of the quote. "How do you imprison a Titan? Funny you should ask…" With what we know from Cold Days, this seems to be a clear reference to Demonreach Prison. With the description of Battle Ground, perhaps this was foreshadowing more than just the purpose of the island? We still don't really know how to imprison a Titan or any other extremely powerful being. We just know where.
There's a lot about Demonreach we don't know about. Why, for example, aren't all or most of the monstrosities in Harry's "green book" in the Prison? Does it have a capacity limit? Can beings be summoned out of the Prison?
