Sure it will have more viewers than your average game in a 1est time slot, but....
How much of that being the only game on is lost to people not wanting to wake up for it? You also lose the church before football crowd which in the south is a massive demo. I know it's a big draw but It can't be anything near any of the other primetime slots even Thursday behind it's paywall I bet pulls better numbers
Edit: found some ratings, they suck lol
Its not as big as a Thursday game. It'll still do significantly better than any individual 1pm timeslot, due to the lack of competition for games.
I'm sure the whole "church crowd" thing might be somewhat significant, but you have that same problem for anybody in Mountain or Pacific time for normal Sunday games, as those games kick off at 10-11am local time.
All of this is also kind of moot, because the overall purpose of international games is to increase international interest. They're going to avoid putting a game in London on at 6pm local time, because it'll draw less than a 330pm local time would.
US interest is already what it is. People who treat the NFL like a religion itself (like I do) are going to watch. I'll just change my plans to do so. I'll skip church, go to a later service, or in my case, not go to church at all.