Ah ha!  He does know

https://www.youtube.com/watch?v=6okRn3oA0w8

at 20:59 he discusses exactly this.  He says that unfortunately Legacy of the Void is coming out on the same day as Fallout 4 and he hasn't had anything like Fallout 4 to play in a while.  Something along those lines.  So, he's going to play Fallout 4 first.