if 100 SEO companies offer optimization service to 1000 companies selling the same "super widget", isn't it logically impossible for them to guarantee (or even ALMOST guarantee) that your "super widget" company can get ranked on the first page of Google for the keyword "super widget" and simultaneously give the exact same guarantee to the other 999 as well?

It is not possible and any search optimization company that guarantees specific placement in the major search engines for natural results (i.e. not paid) is lying to you.