I didn't follow the case but it LOOKS LIKE he posted it unintentionally while trying to add Realism to an online game he was playing.

"While he worked at an Air National Guard base, Teixeira obtained materials including maps, satellite images and intelligence.. and then posted those documents to an online platform popular with gamers.... (It included) confidential information about the war in #Ukraine."

Jack Teixeira: US airman sentenced to 15 years in prison
https://www.bbc.com/news/articles/c62l00g0k65o