He found the address in the source code of the ransomware, any researcher could have found it. He even said himself that when he found it in the source code and saw it was unregistered he registered it to see what would happen. As it turned out it stopped infections from occurring.

Not to say that he isnt the malware writer but your use of quote marks makes me think you have no idea about what happened and havent looked into it, just made some "wild assumptions".

Pretty sure it was in disassembled machine code, not source code.

I have taken the liberty to download a sample of WannaCry and I can see the "killswitch" domain just running strings on the binary.

    $ strings Downloads/24d004a104d4d54034dbcffc2a4b19a11f39008a575aa614ea04703480b1022c.bin |grep .com
   __p__commode
   http://www.iuqerfsodp9ifjaposdfjhgosurijfaewrwergwea.com

An extraordinary claim requires an extraordinary proof.

You don't need to proof anything to raise questions. The comment didn't claim anything, it was just a sceptical one. We should cheer those.

The firm he works for literally pays him to track size and scale of malware outbreaks. Whats the best way to do that? Look for domains the malware attempts to communicate with and register them, pointing them at the firm's sinkhole server. From there the server can generate reports on how many connections it gets and from where.

He did what he would of done to any malware once he found an unregistered domain, he registered it. He didn't realise the malware was using that domain as a killswitch.