I wouldn't say that o-linemen are smarter than QBs, but I will say that of all the positions on the field, OL and QB are the two most demanding, mentally.
The trick with the offensive line is that there's five of them, and all of them must function flawlessly and in unison for a play to work. All five must read a defensive front, determine how to adjust to it, communicate the blocking scheme, and execute it with full faith that everyone is performing exactly as they should. In the 15 seconds or so that you get before the snap, this isn't easy, and requires some really quick thinking.
Of course, compared to what a quarterback is doing at the same time, and during the play itself....
Both are complicated positions, and people who just can't grasp the intellectual side of the position tend to suffer noticeably.