I dont think sky were clean last year, the whole team were cycling on the front without any extra effort. This year several of them cracked which leads me to think theyve stopped taking whatever they were on.
Last year was a flatter route and much easier to dominate as a lot of the so called big names were missing as well so Wiggins was not being taken on, the team as a whole was not as strong this year. Sky took a stand after the Armstrong affair and then a few riders left along with Yates who it was alleged failed a test in 1990's, So I honestly do not think Dave Brailsford would have any of his riders on EPO or anything that is not legal in order to get Sky to win the TDF.