Looking at the timeline for the post, Thaddeus initially closed it as a dupe using his mod-hammer, but it was re-opened by five regular users, including myself, Mike, Null, Jason Baker, and SS.
I see no wrongdoing or abuse of powers here.
Regarding the updated question, I still see zero issue here. The question was determined, by proper channels, that the questions were indeed different. Additionally, the answer posted by Thaddeus was not a copy/paste job or matched answer from the other question (which would have made it a dupe). Once Thaddeus (or any other user) understands what the difference between the questions is, I see no reason why they shouldn't be allowed to answer the new question as they see fit.
Regarding the moral issue of answering a question you've previously voted to close, so long as the user is not still arguing that it's a dupe/off-topic at the time of answering, I still see no issue.
People change their minds. Having previously thought an opposite way should in no way preclude a user from writing an answer at a later time. Even if they haven't changed their minds, I still see no problem. This is what appears to have happened here. Thaddeus closed it, users explained why the closure was wrong, users voted it back open, and he answered it.
If the voting user was still obstinately arguing that the question was a dupe/off-topic, and wrote an answer anyways, I'd see where some users may be put off by their behavior, but if a user wants to write a useful answer, I see no reason why they shouldn't.