You don’t seem to be acknowledging the fact that the goalkeeper’s positioning and - more importantly - setup of the wall essentially determines what kind of shot the free kick taker can go for. Most of them completely shut off the far post and design the wall so that it has to be a worldie to get into the near post post. Henderson didn’t do that at all. He didn’t have his near post covered, nor did he have the second wall effectively blocking it. Instead he left a spot wide open that neither he nor his wall was covering.
That makes the decision really easy for the kick taker; you don’t need to disguise anything, you don’t need to worry about obstructions, all you need to do is focus on a clean strike into the most direct route to goal. It’s not like he had to get it around the wall. It was nowhere near any player or the keeper. And not only did Henderson not realise the gaping hole, but he doubled down on that error by moving even further away from it.
The reason all this happened is because he has such poor reach that if he covered his near post properly, he’d never be able to get across to the far post. So he leaves bigger gaps than the vast majority of keepers do there, in both situations. And he doesn’t have the reflexes or footwork to recover those situations, nor the positional and organisational sense to mitigate those situations.
Any set piece taker in the Premier League can hit a sweet strike into the far corner, unobstructed. They won’t land it every time but it’s not at all unusual to see someone execute like Maddison in that situation. What is unusual is that scenario even presents itself. Free kicks are the ultimate test of a keeper’s ability to anticipate and obstruct the opposition. They get all the time in the world to plan out the scenario, and essentially no time at all to recover from the mistake.
De Gea has saved a lot of “unstoppable” free kicks in his time. That one against Mata that everyone loves, he dictates the situation from start to finish. He covers the far post so there’s no point in Mata even trying, and at the same time he has the footwork and reach to get back to the near post. It would’ve went in off the post too, and if the keeper gets nowhere near it, it’s called “unstoppable”. He‘s done that loads of times. Henderson on the other hand has loads of examples of judging his angles poorly, and giving himself no chance to make a save once the striker takes the obvious choice.