Leaving aside the point that I made above about what "half-life" actually means, Marcus's statement is odd in another way: What on earth would the British gain by making up a lie that Po-210 was used to kill Litvinenko?If Russians had wanted to kill him, they would have done it in another way. He could not be poisoned by polonium, because its halflife is 140 days.
Granted, most of the world's Po-210 is produced in Russia, so if Po-210 was indeed the poison, that tends to raise suspicion of Russian involvement. But (1) the use of Po-210 doesn't PROVE Russian involvement; and (2) Russia would've ALREADY been a logical suspect, because (duh) Litvinenko was a defected Russian agent, and had also made inflammatory claims that Russia was blowing up its own apartment buildings in order to frame the Chechens, etc.
Thus, lying about Po-210 would seem to be unnecessary (because Russia would already be suspected no matter what), while at the same time being insufficient to conclusively blame Russia (because the Po-210 could have come from another source). So such a lie would accomplish nothing, in my opinion.
Moreover, Litvinenko's symptoms were generally consistent with thallium poisoning -- in fact, this was the original diagnosis, and polonium wasn't confirmed until his autopsy.
But if he had been indeed poisoned by thallium, making up a lie about Po-210 would create a totally unnecessary and expensive public health crisis for the UK -- because the sweat, urine, and feces of a polonium-poisoned individual are more hazardous to hospital workers and acquaintances than the bodily excretions of a thallium-poisoned person. Also, the general public is just scared as hell of radiation, even when there's not actually enough of it to harm them. (For example, the trace amounts of polonium-sweat that Litvinenko left on the seat of a London taxi would arguably pose very little danger to other people using the taxi, because your underwear and other clothes would block essentially all of the radiation!)
So, in short, if you falsely tell the public that a thallium-poisoned person was actually killed by polonium, you have to spend a lot more money on decontamination efforts. And for what gain? If the point was to frame Lugovoi for something he didn't do, planting a vial of thallium in his hotel room would have done the job nicely, without creating a public radiation scare!
Finally, making up a false story about polonium would require a relatively large conspiracy in which a lot of ordinary police and doctors and other investigators were paid off to lie about having detected polonium in Litvinenko's body, and in the blood of his wife and other associates like Mario Scaramella, and in various restaurants, taxis, and hotels that Litvinenko had visited, when they actually hadn't detected polonium anywhere or in anybody.
And if you're MI6 and you're trying to kill a former Russian intelligent agent while framing Russia for the murder, you naturally want to involve as few people as possible in your conspiracy!
So, while different people can disagree about who poisoned Litvinenko with Po-210, it seems to me literally unbelievable that polonium wasn't the substance used to poison him.