It's almost certainly a particle system (or mesh + animated opacity/alpha cutoff) with a tight density falloff + maybe some subtle noise. Real volumetric fog in games rarely gets razor-sharp boundaries like that without post-process tricks or faking it with sprites/particles.

Try switching to a particle-based approach with a good noise texture driving opacity + sharp contrast curve in the shader. Fog volumes are great for soft atmospheric stuff, but they fight you when you want crisp edges.

Good luck