I dont read much in the article to make that idea exceptionally plausible.
None of the individuals noted seem to have any reason to associate with terrorist with possible exception of Litvinenko
The only possible scenario to fit this idea is that Litvinenko contaminated himself while dealing for the Polonium and lightly contaminated others while traveling around.
To make the theory fly they will have to connect Litvinenko or one of the others to terrorist.
"To make the theory fly they will have to connect Litvinenko or one of the others to terrorist."
Litvinenko was a Muslim, but I don't know the details of his conversion. I had read somewhere that he had extensive contacts in Chechnya. I think this story is going to get very interesting in the next month or two.