I do agree with the sentiment. It is extremely difficult and unlikely that a great shot will be saved.
But as long as we're talking about impossibles, we should also mention that it is impossible that a penalty taker, regardless of how well trained he is, can make the absolute perfect shot every time. Being well trained is definitely a huge leg up, but after a certain point, other factors can make a bigger difference than being 2% better trained.
If Hart had accidentally decided to dive the other way, we wouldn't be talking about how Messi needs to practice more. He didn't miss. Hart saved it, because he guessed the side. That's a significant difference.