Over the years from 1800 to 2005, "in the list" has, with the exception of one brief down period between 1975 and 1985, been at least a little more common than "on the list" in all occurrences tracked by a Google Books search. Here is an Ngram chart for "in the list" (blue line) versus "on the list" (red line) for that period:
There are many complicating factors at play here, because "in the list" and "on the list" can be used in a number of ways—including some that are not relevant to a written list of items. But I also created an Ngram chart for "item in the list" (bkue line) versus "item on the list" (red line) versus "items in the list" (green line) versus "items on the list" (yellow line). Here are the results:
The upshot of the second graph is that even when we focus on a much more narrowly defined form of "in the list" versus "on the list," we get multiple matches for both forms—and in this case, it happens again that the "in the list" forms are generally a bit more common than the "on the list" forms—at least in recent years.
I looked at several pages of search results for "item in the list" and "item on the list" from the period 2000–2005, and the only really prominent difference that I could detect was that computer software texts seem to strongly favor "items in the list"—and even among such texts I found some instances of "items on the list" as well.
My conclusion is that both "As promised in the list" and "As promised on the list" are reputable ways of expressing the same idea. John Green is not wrong to choose in for his sentence, nor would you be if you chose on instead.