From the article: "Air Canada told Go Public in an email there is no conclusive medical evidence specifically linking deep vein thrombosis with flying."
A very easy position to take, as long as there is a single dissenting voice. Heck, we're still debating whether smoking causes cancer. AC has seen fit to mention DVT on its website in the 'special needs - medical' section. It doesn't go so far as to say flying causes DVT, but I interpret its inclusion as a tacit acknowledgment that sitting for extended periods at the very least exacerbates a pre-existing condition. It's a stretch to claim there is no link between flying and DVT, unless we intend to be hyper-literal. Sure, it ain't the flying that causes/worsens DVT, but sitting in horrid Y seats is certainly a factor for those at risk to get it - regardless if there is air between the wheels and the ground.