That's a writeup on the report from the Chernobyl Forum, a group created by the UN's International Atomic Energy Agency. The WHO was a member of the forum, and it won a Nobel Peace Prize, but there are some large issues with their projection methods.

That said, mine wasn't peer reviewed, and used the same method as the Forum report with some small tweaks. I didn't check my sources well enough, but I also can't find anything credible listing it far above that number.