It is the combination of his technique, strength and instincts. Seymour is quick off the ball, stays low (watch his pad level, even though he is 6'6'', leverage) and gets his hands underneath the shoulder pads of o-lineman and uses his powerful and long arms to keep the o-lineman off of his body (giving him the ability to utilize his strength, and technique to maintain the line of scrimage, and play a 2 gap technique). This allows Seymour to completely control 2 of the holes on an offensive line, effectively taking away one half of potential holes in the offensive line.
His ability has a residual effect, in that it protects the other D line man and linebackers, as it is virtually impossible for an o-lineman to trap him or scrape off him for down blocks. The result of this is that the o-line does not get a hat on one of the linebackers, or a double team on one of the other d-linemen, leaving them free to make a tackle, or puts one of the linebackers one on one with a runningback.
His statistics are not fantastic because he is always double teamed and one of his teamates will make the tackle because Seymour held up the o-line.
Also I would say that Seymour is a lot faster than he is given credit for. He is a long strider like McGinest so he does not look fast (smaller quicker steps looks faster) even though he covers more ground per step than most.
If you watch film on him, every coach and player in the NFL knows how dominant he is.