Why does Y!A now allow one to answer a closed question?

It depends which version of Y! Answers you are using.

If you are using the old (green) version then you can't answer resolved questions but yes I also tested and was able to answer a question which was assigned a Best Answer already as well http://answers.yahoo.com/question/index;_ylt=Agyas...

Glitch? Deliberate new function? Who knows to be honest with the new version. Information is not exactly forthcoming from the Yamster which is a shame and therefore we are left to draw our own conclusions.

You can also comment on everyone else's answer in the new version of Y!A but cannot comment on the question or your own answer (e.g. to say thank you or to answer any further comments placed in the BA comments).

It's going to take quite some getting used to by the looks of things.

There must have been some type of temporary glitch that allowed the "Answer question" button to show up. It did not show up for me.

Interestingly enough, when I first clicked that link you posted, your answer showed there, with a time that was obviously AFTER the Best Answer was chosen. Now, your answer is NOT there any longer. I also got some VERY odd links showing in my "history" as a result of clicking your link, things with the word ADCLICK or something like that in them. Just a coincidence, I'm sure.

I don't think it does - there's no sign of any answer by you.

It could be "Net lag" - a glitch in propagating the Q&A between different Y!A sites. If there's a fault in that, there could well be a short interval when a question is closed on (say) the UK Y!A, but still open on the US site.