So I read the article and I don't think it's as bad as you make it. The part that explains his involvement in the investigation is actually backed by a now inaccessible source. It would be crucial to know what the authors linked there before we judge the credibility.
The rest is a matter of opinion. They obviously have no definitive answer, so it's all speculation. Their point is that his personality changed after these trips.
Maybe I would be more inclined to agree with you if I hadn't seen a Channel 4 documentary on Litvinenko [1] where one of the Scotland Yard detectives recounts lots of strange things happening on their separate trip to Moscow: parts of their group fell ill, they thought they were being followed, and they were driven aimlessly around before they could see one of the suspects.
It's actually pretty easy to figure out what document was linked to explain his involvement in the investigation. If you look at the URL of the now-inaccessible document it ends with INQ007647. That's the identifier of the statement to the Litvinenko inquiry describing the late 2010 reassessment of how much radiation Litvinenko was exposed to based on the new discovery that he'd been exposed twice, which it specifically mentions Dr. Puncher was involved in. I linked to the copy of that document on the inquiry website in my original comment. It wasn't just a case of Buzzfeed linking to the wrong document either; I looked at the earlier documents which it was a follow-up to and the earlier work was all done by other scientists.
Thanks for your explanation! Since HN cut of your URLs, the similarity between the filenames didn't jump to my eye. You're right, 2007 doesn't mention Puncher, it only mentions his close colleague Birchall. Both were publishing articles together around the time [1-4], so I find it hard to believe that he wouldn't work on such a high-profile case that was being solved by their computer software [2]. Since Buzzfeed authors talked to Birchall and Puncher’s family, I trust their accounts of his involvement.
The rest is a matter of opinion. They obviously have no definitive answer, so it's all speculation. Their point is that his personality changed after these trips.
Maybe I would be more inclined to agree with you if I hadn't seen a Channel 4 documentary on Litvinenko [1] where one of the Scotland Yard detectives recounts lots of strange things happening on their separate trip to Moscow: parts of their group fell ill, they thought they were being followed, and they were driven aimlessly around before they could see one of the suspects.
[1] http://www.channel4.com/programmes/hunting-the-kgb-killers