The term “warfighter” appears to be originally a US Department of Defense term used by analysts when discussing or analyzing warfighters (and how to best train them, equip them, support them, deploy them, etc.).
The term has utility because a) it includes the members of every branch of the armed forces, including soldiers, marines, sailors, and airmen, b) it distinguishes between those who fight in a war and those in a support capacity, and c) it is gender neutral. Unfortunately, as with many words in language, its use has been “expanded” and used in a variety of ways not associated with the original meaning.