You can't go from one battle to another on the same map. You have to change servers. It's not a global war, it's a centrally maintained stats database that updates the conflict on each server to make it seem interlinked.
The day you can go from fighting in one area and fly to another to make an impact on that battle is the day it will cross to MMO.
Go on moving those goalposts, boy.
Even if it indeed only is a centrally updated stats database that decides which battles get played when with what equipment that's still a persistent world and qualifies as an MMO because the actions of all players across all maps affect the global situation. Meanwhile even a 256-player BF or IL-2 COD server can never qualify as an MMO because none of the necessary systems needed for persistent world gameplay is present in either game.
And no, Stalker wasn't an open world. It tried it's best, but it still had boundaries for both the player and the NPCs. You could always expect one group of critters to be at the same spot every time and they were restricted on movement.
Funny how all 3 Stalker games I have played from start to finish have had dynamically generated NPC's and critters engaging in impromptu battles, or how a corpse on one map is still there when I return to that map. Saying an open world game cannot have player boundaries is plain dishonest. In that case GTA isn't an open world game either. Oh look, 99% of gamers say it is one.