I looked there as well and it's nowhere to be found. It's disappointing. I can airplay it but I'd rather play it directly from ATV to save iphone battery.
Hoping for the same. I tweeted them (@OvercastFM) to request an ATV4 app. They seem to have replied to someone else that they have some other things in queue first, but hopefully they'll do one. Just discovered Overcast on iOS a couple of weeks ago and like it a lot better than Apple's app....and it allows AppleWatch control.