Of course, the one thing that gets ignored in such analysis is launch angle. Britton has one of the lowest launch angles against him on groundballs of anybody in MLB. So even though the ball is coming off the bat hard, it's not necessarily perceived as a hard-hit groundball because there are less 1- and 2-hoppers. On a somewhat related note, because of the velocity he has a smaller percentage of groundballs to the pull field against him than average and a huge proportion of groundballs hit back towards the middle of the field. Not a lot to get through the holes between 3B and SS or 1B and 2B.
I'm not saying his BABIP isn't a little bit lucky. But I also think that the expectation value should not be considered to be the league average on groundballs, or even particularly close to it.