Umm, could you link to that maybe?
Even though they aren't claiming these claims are true, its at the very least, incredibly misleading. They spend most of the article talking about how Putin has poisoned people in the past, and talking up the Drs credentials. There should be a disclosure that the claims are unsubstantiated. Or it should never have been posted.
That said the Washington post did some of the best reporting this election season.
It's true that headlines like these cause people to become certain about what is an unsubstantiated idea. But it's also within the realm of possibility that Putin did try to poison Clinton. It would be far from the most egregious thing he's ever tried to do, and (as the story describes) he's done it before.
They didn't report it as fact. They reported it as, "This respected guy is saying this thing."