Your right it probably was not the dirtiest tour ever but it's up there, as a contender and thats why I say arguably. What it does have in it's favour is first of all a 37 yo Armstrong hailed as comeback hero while making a micky out of antidoping on the podium. This, unlike his 7tdf wins, is after several revelations have emerged about doping in cycling, about him, about his team, and still he strolls in praised by the sport as a savour. and he's 37yo.
Secondly it has a popped winner, ok many tdfs have that but I'm not saying it wins these categories just that it holds it's own. It also has 3rd 5th and 6th place popped and the kom.
So less than other tours but having said that we need to take into account that all the tours it is being compared to have had far more time to identify the dopers. Armstrong fell 7-13 years after his victories, Levi with him. Riis fell 10 years after his victory. So 2009, which has only had 4 and a half years pass is pretty shady with already 4 of the top 6 down and one of them is Andy Schleck who stands on a technicality.
Finally the joker card the 2009 tdf holds is verbier, Vam record fastest ever, with ventoux the fastest yawnfest ascent up a mountain ever, playing a supporting role. the fastest anyone ever went in the epo era was a doper in 2009. That's important. And together with the other things on which it scores high, a reason why it holds it's own in the DOAT argument.