There was absolutely nothing wrong with Wagner's tactics. Every over he bowled two bouncers, two full ones outside off, and then two at the ribs. That's not illegal and it's not bodyline. That's just trying to get him to top edge one to fine leg. There's nothing wrong with that tactic, the West Indies did it all the time in the 80s. Every time they found a batsman was compulsively playing the hook shot, they'd bowl some short stuff from round the wicket and see if they'd bite.
Bodyline was every single bowler hurling down six fast deliveries an over (or rather eight in those days) from round the wicket at the armpit or head, with five-seven fielders on the leg side, including several behind square. At every batsman. Plus - no helmets or chest pads. The only way any batsman could score a run was to hook it for six, and if he missed it he would have ended up in hospital with concussion. That's why it was banned - it provided the batsman with absolutely no opportunity to score runs and it was dangerous to safety.
Smith never had more than two fielders behind square leg. At worst, he had a fine leg, a leg gully, a man standing just in front of square leg, and a silly mid on. He could have waited out the two bounders, let the two armpit balls go, and then scored off the other two fuller deliveries. Or, it he had a better technique, he could have found the boundary regularly. If Ricky Ponting had been facing that stuff he would have dispatched most of it to the fence in front of square.
You might notice that they didn't try the tactic to most other batsmen. They never tried it to Burns, or Labuschagne, because they both play the pull shot well. They also used it sparingly to Head, Warner and Paine. But they confused the hell out of Wade.
For Smith, it's a technical weakness - he struggles to keep the pull shot down to left arm bowlers (or right armers from round the wicket). He's always looking to walk across to the off side to play that front foot flick through leg, so his bat starts low. When the bowler bowls it short, he has to bring the bat upwards from low down, almost guaranteeing he'll hit it in the air. Four times he hit one to backward square leg - and none of them were unplayable balls.
It's a technical weakness and Wagner found it.