This is utter nonsense. He didn't stop Wannacry. The Wannacry devs stopped Wannacry, if he didn't grab the domain it'd have been picked up by some other TI firm within minutes or hours.
Just because someone else could have stopped it, doesn't mean he didn't stop it. That's... just a fact...
This was inevitable because of how WannaCry was designed.
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.
Registering unregistered domains the malware connects to is a very obvious thing to do, in this case he got "lucky".