The implications of Ebenezer potentially time traveling are absolutely ludicrous, especially because he's the Blackstaff.
Personally, I chalk this up to Jim not having his ducks in a row and making an error, either in the book or when answering the question.
...but if he's right in both places, then time travel is on the table as an explanation (as is temporarily picking up the Blackstaff, or some other explanation (he helped the Blackstaff of 1811 pull it off in some way without violating the Laws, like watching his or her back as they built up the geomancy spell, which tend to have long casting times and are nonideal for fast-paced combat)).
But if he does travel back to cause the New Madrid earthquake, then that means Harry is currently living in an altered timeline. Though that tracks with some of my own IRL ideas about time travel-based timeline corrections. Personally, I think that if time travel to the past is possible and it can be used to alter the timeline without spinning off alternate realities (like "Go back in time and kill Hitler,") then there is, essentially, a 100% chance that we're living on an already-"corrected" timeline. That means that this is the best possible outcome humanity of the future can conceive, which is oddly comforting, if you think about it.
But if Eb risked a universe-shattering paradox by going back to cause the New Madrid Earthquake in 1811, that means that there was something worse going on. Something so bad that chancing oblivion was worth it.
...though I'd also like to point out that New Madrid is a fault zone, and has had multiple earthquakes over the years, including a 6.0 quake centered on a major population area in 1895. I think it was Charleston, MO, but someone correct me if I'm wrong. So, Eb might've been talking about one of those—but if you ask someone about the New Madrid Earthquake, they all point to the one in 1811/1812, so that could be wrong, too.