I don’t 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.
From Wikipedia:
"In a July 2005 interview with the Polish newspaper Rzeczpospolita, Litvinenko alleged that Ayman al-Zawahiri was trained for half of a year by the FSB in Dagestan (a republic neighbouring Chechnya) in 1998.[17][18] According to FSB spokesman Sergei Ignatchenko, Ayman al-Zawahiri was arrested by Russian authorities in Dagestan in December 1996 and released in May 1997.[19]"
And concerning his conversion:
"Two days before his death Litvinenko, a disenchanted Russian Orthodox Christian, informed his father that he had converted to Islam: the actual conversion happened at a point during his sickness but before he knew he was going to die.[33][34] Akhmed Zakayev, Foreign Minister of Chechen government-in-exile who lived next door to Mr Litvinenko and considered him "as a brother",[35] said: "He was read to from the Qur'an the day before he died and had told his wife and family that he wanted to be buried in accordance with Muslim tradition."[36] According to Vladimir Bukovsky, Litvinenko accepted Islam mostly to show his solidarity with Chechen people, who he felt were brutally oppressed by Russians.[37] Before his burial, prayers were said for Litvinenko at Regent's Park Mosque.[38] Litvinenko's reported conversion to Islam and the related wish for Muslim funeral rites were recognized by his father, but have been rejected by his widow, Marina, as well as his close friend (and press spokesman during his illness), Alex Goldfarb who preferred a non-denominational ceremony.[39]"
So my questions are these:
Did Litvinenko know/have communications with Zawahiri during this time?
How long ago did he become so enchanted with Islam?
Did he have some sort of admiration and empathy for Chechen terrorists like some of those who feel admiration and empathy for Palestinian terrorists?
I don't know exactly what it is, but something doesn't fit about this story.