That is very strange. I see the PR but when I search on coinbase, there is absolutely no mention of the segwit2x on 12/29/17. One would think coinbase would at least mention it even if they don't have a plan yet. They did end up supporting bitcoin cash. I don't know what they are going to do about bitcoin gold.... But I thought coinbase was a strong supporter of the segwit2x when it was originally suppose to happen in November. I would think they would have supported the December fork.
I own a lot more bitcoin now than I did in August for the bitcoin cash fork so I am trying to get all the details I can on it.