Arma: Cold War Assault Remastered out with a demo and it's open source

I am actually amazed that they open sourced it.

ARMA, old ARMA, Op Flashpoint, is jank as a fuck as an engine, when not running on a supercomputer, but also… it does some pretty insane shit in terms of detail and scale and number of AI/NPC agents active at the same time. And we basically all have ’supercomputers’ now, by the standards of when it originally came out.

… there could be some interesting patterns/techniques in here, maybe not to emulate or use exactly as is, but try to learn from, if one is insterested in having a game that is basically also a big simulation, in 3D space.

Is it normal for Arma games to be FOSS?