This was an intentional change, part of a larger set of changes to the whole question closure mechanisms. And I think that it is a good change.
Closing has a negative association, and while the intention behind it is not necessarily negative, it will be viewed in most cases as negative by the user who asked the question that is closed. There are factors that exacerbate the negativity, e.g. a non-obvious closure without a comment explaining why, or sarcastic to even hostile comments on the closure. But even in the best of circumstances, the word closure will be perceived as negative, the user that asked the question will get the impression that he/she did something wrong.
And while sometimes the user did something wrong, and did not even do a basic search before asking, often the duplicates are not obvious or easy to find. The idea is to go from the negative point of view, of telling the user that they did something wrong and that we shut down their question, to a positive view. Closing as a duplicate means that there is already an answer to the users question. So now we are telling the user "look here, we already have an answer for you". The change of "closed" to "marked" is one of the changes that are meant to give new users a better impression of what it means when their question is closed as a duplicate.
There is no practical difference between "marked" and "closed", this is purely a change of name.