I saw that wikipedia listed them as prefab, but I think it is incorrect.
Kit home: Everything you need to build a house is in this box. Just add labor. Sort of like buying a lego kit that you know has all the pieces to build a Millenium Falcon out of.
Pre-fab: Important/time-consuming elements are constructed off-site and transported in a (nearly) ready to assemble state to the site. Sort of like having the Lego factory assemble the 7 major parts of the Millenium Falcon, and sending those to you so you can just connect them to each other to have the final product.
As I understand it, the Sears homes were kit homes by those definitions, not prefab. E.g. you would receive a bunch of 2x4s in a railroad car, not a ready-built wall you could just raise into place. Kit homes can also be pre-fab homes, but I don't think the terms are interchangeable and not all kit homes are pre-fab.
But I appreciate the distinction you are trying to make.
Sears homes did include extensive, hand-made details such as hand-made stained glass, cabinetry etc. So it may be a grey area and not perfectly match up with the vocabulary of today.
And people now pay a lot of money for those prefab houses:
https://en.wikipedia.org/wiki/Sears_Modern_Homes