Romney does get that the federal government can not be used in that way, he has stated it and even explained that in the video you responded to.
Again how does Romney believing in the 10th ammendment create irony?
Because the tenth amendment isn't the only issue here. The more important issue, in my view, is personal freedom. Romney clearly doesn't see anything wrong with using government to order people to buy things they don't want. Whether it's technically constitutional or not, it's just as much a violation of our rights at the state level as it is at the national level.