Hemingway wrote standing up because he had a serious case of hemorrhoids. Sitting was painful for him. I don’t know how much this influenced his writing. His prose is beautiful but I don’t think he had much to say that’s still significanlty relevant.
As Howard Stern said, proper wiping prevents hemorrhoids so what does that say about Hemingway?
Covering WWII Hemingway was already famous so it was sometimes was a drawback on what he was able to cover. Ernie Pyle was a reporter who became famous for his coverage in WWII. So sometimes Hemingway would introduce himself to the troops as Ernie Roid, the poor soldier’s Pyle.