Vasily Smyslov was a positional player at heart. Like all of the greatest players he could spot a tactic as quick as anyone, and play it if it made sense. He did not seek out that type of game though. He was like I'm going to play 40 good moves. If you play 40 good moves we will draw.
He wasn't going to be reckless and gamble with razor sharp, deep tactical play. He was going to simply play great chess. If you could not play at his level he was going to punish every slight inaccuracy. Very few could play at his level.
You can rest assured all of his contributions to chess theory are going to be positional in nature. Playing for small advantages. Sometimes abstract elements that would evade the untrained eye. Smyslov knew how to nurture them and make them grow.
His move here 3...g6 preps the Bishop for the long diagonal and concedes no structural weaknesses. He answered 4.c3 with 4...a6 putting the question to White's light square Bishop. If you face this system with White, try instead the immediate 4.d4 exd4 5.Bg5 and see if you can make anything of the resulting pressure. Remember you're playing against the Smyslov Defense, not Smyslov!