Currently, the game’s original creators are working on “Uncharted 4”, but of course this doesn’t mean that they can’t split up their attention across multiple different titles, especially if one of them is as important as this.
On the other hand, it’s also possible that the game is still in the initial stages of planning and they aren’t actively working on it, in which case it will be a while before we get any official news.
That could explain the attitudes of both voice actors in the situation, as it’s possible that the game really is in development and there are rumors going around, but not everyone has been notified yet and the voice actors themselves might simply not know when they will be called in for work.
In any case, the announcement of a second game in this series is going to be huge news on the Internet, and it will be talked about a lot. We don’t know when the game might be shown to the public, but we certainly hope that it’s going to happen sooner, because fans are already excited enough from these rumors. We can only imagine how forums will look once it’s been officially confirmed.