What about 3D modeling is inherently linear? Manipulating/translating points is a likely a matrix transform. (e.g., http://en.wikipedia.org/wiki/Transformation_matrix#Examples_in_3D_graphics ) Maxtrix transforms can be parallelized. There is a difference between legacy assumptions built into the code and a requirement for serial, linear processing.
No most modeling apps are NOT well threaded - if at all. DL some free trials and try it - you'll see. Rendering is different of course...