Isn’t it the case that “wonder about” would be followed by a noun or noun phrase and “wonder” followed by a clause?
In your example the word “that” assumedly replaces an entire clause. AND it sounds like someone else has just said something like “I wonder why John hasn’t arrived yet”, so when Langdon says “that”, he would be specifically referring to an “I wonder why” construction. It would seem logical for Langdon to then use “wonder that”.
But I think if Langdon were thinking about some clause or sentence simply as “that topic”, then saying “wonder about that” would be OK.