Mohler discusses these ideas in his daily radio program from last Wednesday as a response to a caller. The question begins 18:35 into the program, and the discussion continues for about six minutes.
The short story is that Mohler acknowledges "a real danger" in both fundamentalism and evangelicalism. With evangelicalism, before long "you can't tell the difference" between liberals and the evangelicals who cooperate with them. With fundamentalism, "you wind up talking to yourself" because you have no fellowship with people who share the same beliefs but are in liberal denominations.
He says that fundamentalists "think they're more separated than they are," which raises an issue with which fundamentalists need to grapple, even though Mohler explains it awkwardly, illustrates it poorly, and does not acknowledge the difference between merely existing in culture and willingly cooperating with unbelief.