There was a puffpiece written a few weeks ago where some staffer working for the Oppenheimer film stated that christopher nolan said, that he didnt like the two movies coming out at the same time. Who knows if this is true.
From there a bunch of forced probably fake drama has been floating around on the internet, where tabolids are trying to create a rivalry between the movies. Even ryan gosling and other unrelated celebrities commented on it. Most just stated that people can see two movies in one weekend.
Ultimately its probably just a marketing technique.
For me, i Just really like the idea of barbenhiemer.