I mean, he literally stopped WannaCry.

Just because someone else could have stopped it, doesn't mean he didn't stop it. That's... just a fact...

You're mixing up could and would here.

This was inevitable because of how WannaCry was designed.

If Newton had not discovered and articulated his 3 laws of motion when he did someone else WOULD have in the decades that followed. Is it an error to say Newton was the first to discover his laws of motion? By this definition how can anything be attributed to anyone?

Do you attribute a car bombing to the person that set up the bomb or the guy that inevitably turned on the car and set off the bomb?

In this case WannaCry creators built a system that would inevitably be triggered within a few hours from the malware going live.

I think it is meaningless to attribute the WannaCry killswitch to him instead of the authors. If he hadn't registered the domain some other threat intelligence firm would've done it moments later.

I mean he had to notice that it was contacting that website, might not have been an obvious thing to look for.

Nah, this was one of the first things you'd notice. No need to reverse the binary, you see the domain in pcaps when you run it.

Registering unregistered domains the malware connects to is a very obvious thing to do, in this case he got "lucky".