Fundamentalism might not have been the main theme of the KKK in the 1920s, but it was certainly not that different from what they were advocating. During this time, the KKK was promoting what they saw as traditional American values -- just like the fundamentalists were.
Both the fundamentalists and the KKK were fighting against the new ways. Both did not like the new ideas that were springing up in the cities. Because of this, it is not wrong to put the KKK in with the fundamentalists, at least to this extent.
While it is not right to say that all fundamentalists were KKK members, they did share some goals -- they both wanted to return to the values of the past because they did not like the flappers and immigrants who seemed to dominate the cities.