You can find goals like that in every competition. That was just one goal, in the whole season, for all we know he was just lucky... He has to show things like that more often to get a label of having a great shot in him. Like you said, he luckily doesn't need that due to being such a quick dribbler that he barely needs to take a shot from afar.
There's a reason why Rooney, Mata, and Memphis are always behind the ball for freekicks. They got it in them, they've shown it at training, and on the pitch, multiple times. For the same reason it was Martial who took a penalty last year, he can keep his cool, and just finish it.