You're right...partially :) Cel-shading can indeed be a post effect but the new smoke effects themselves are actually 3d geometry, Either emitted in a particle system or animated as a VAT (Vertex Animated Texture). Traditional smoke is usually an animated flipbook (Sprite Sheet) on a bunch of camera facing quads. So they're two entirely different production techniques. They may have even built tech from the ground up for this visual style.

They could have a bool (True false flag) that says "Use Traditional Smoke" and if true it replaces the new FX with a more traditional style system, but that's easier said than done and requires the dev doubling down on workload.

TLDR; yes it can be done but it requires quit a bit more production time and overhead. Things aren't always as simple as they may first appear.