That's not what he said in the last few interviews I read and listened to. Several times now, he has straight up said that Walker is dead the entire game and trapped in a sort of purgatory. I've been following this game pretty closely since finishing it and I have yet to see him state anything to the contrary. Some things are left open to interpretation, sure, but the story was meant to be taken a certain way.
Here's one of the more interesting interviews I've come across: