This is after Scott had gone through Dark Horse's upcoming plans and then introduced Joss:
"And Scott was a little self-effacing there. He is actually - beside the fact that we've worked together for twelve years and I have seen him, almost like with a razor blade to his wrists so many times because of my hilarious cavalier attitude on deadlines... [audience laughs] He, besides supporting me and guiding this whole thing, Scott is actually, this last arc that were doing, he's writing it with me.
"And he has made it so much better, because... I've been busy. And he literally came down to LA, and the two of us sat down for a couple of days just mapping out everything. Making sure everything was working, and everything was going to pay off the way we wanted it to. Emotionally, and in those tiny little details that a lot of people care about way more than emotions. [audience laughs] Yeah, you know who you are. [more laughter] Some of you are in this room.
"And so we're going to be doing that together, and I just want to thank Scott for being up and helping me. It's going to be really, really cool."
In other words, it doesn't sound like Joss has simply handed off the most important issues of the season to someone else to write, as some people apparently feared. It sounds to me more like the sort of thing that went on on the TV show all the time. Scripts were often produced by a group effort, and if an episode came up short one of the other writers might be drafted in to write an extra scene or dialogue to extend it. People who were criticising S8 for lacking this collaborative touch should be rejoicing at this news. :-)
Often the extra work on a script wouldn't necessarily be credited to the writer; but at Comic-Con Joss chose to give a public acknowledgement of the help Scott has given him, by naming him as co-writer rather than just an editor. It was a generous gesture which seems to have backfired on him a little.
It also sounds like Scott has been nagging Joss that he can't just brush off all the details of plot and continuity in favour of emotional impact like he normally does, because the fans will hate it - and Joss has begrudgingly taken the lesson to heart this time. :-)