It's PG-13 because he wanted it to be. Or rather what he made ended up that way.
The idea that WB is yanking this guy's chain to soften his approach solely for ticket sales doesn't even really hold water outside of a basic, generic cynicism regarding a vague idea of how movies are made, which can and does fit in some ways some of the time, but sometimes doesn't apply at all.
The bad conventional wisdom people are using to kitbash an opinion together on this is just making for wobbly, short-sighted opinions coming from multiple flawed premises:
War movies have to be bloody
War movies have to be real
PG-13 is every studio's automatic aim
Directors naturally work at an R
Serious movies that aren't R were neutered to get there
People still give a shit about ratings like it was still 1988 and we're getting away with renting Predator from Blockbuster.
People don't give a shit what Dunkirk is rated. Ratings increasingly don't matter. And graphic violence isn't a necessary element in the construction of an effective war movie
If any director in this industry, specifically any director working for THAT specific studio, has the green light to do whatever the fuck he wants for 100 mil and a handful of 70mm cameras (and I bet THAT was actually more of an eyebrow raiser than the fuckin rating was) it's Chris Nolan.
Dude made what he wanted to make.
What he wanted to make ended up at PG-13.
It's not a cynical production conspiracy on the part of "Hollywood" cutting up your steak into little cubes for you.