this difference engine model has a lot of obvious flaws - the gears do not mesh with each other, the teeth directly overlap. the notches that are supposed to articulate to ratchet aren't actually engaging the teeth on the ratchet. the ratcheting mechanisms on the right are straight up just hovering in the air and are not attached to anything.

that's probably what people are talking about when they say it can't do CAD?

Complex assembly mating is hard to do even in SolidWorks. It's a demonstration model to explain how it works, not a kinematic simulation, turning three.js into a physics engine is beyond the scope of this project.