Interestingly, the source given for the "fatal levels" (footnote 20) appears to be the book "Bugging In and Bugging Out", which is a survivalist guide:

https://www.amazon.com/Bugging-Out-David-Crossley/dp/1291390...

Whether such a book is a reliable source for that kind of claim is certainly debatable, but even more interestingly, the cited page does not even say anything about "fatal" levels of radiation:

"Although radiation levels in Pripyat have now dropped sufficiently for tourists to be allowed to visit the city, ..."

https://books.google.de/books?id=9NlCCwAAQBAJ&pg=PA59&lpg=PA...