“[My trainer] Ray Longo actually broke a guy’s leg like that in the gym by putting the knee right on that shin when he kicked, just by following [the kick] up slowly,” Weidman said. “It’s not really going shin-to-shin, but getting your knee on the shin."-Weidman
“When he landed that kick on my knee, I knew I had checked it [well],” said Weidman. “I thought he would at least be in pain, but when he put his foot back down, and I saw his leg roll up, I knew the fight was over and he wouldn’t get back up. It was a crazy ending.”-Weidman
Checking a leg kick has a purpose. Breaking the leg isn't always the purpose, I give you that. But I think "crazy ending" is more accurate than "lucky ending".
You do have a point in that it wasn't luck that he checked the kick and the intent was to hurt the opponent, but perhaps there was luck in the results.