I don't know that I agree with your assessment that "designers are the core" of Apple's business, but even so, Apple's current state seems to be rather irrelevant to a discussion about a combination of founders for a startup. Wouldn't looking at Apple's structure when they were a startup be more useful?

Aside from that, there are a lot of founders who have some level of design skills -- they just also happen to do other things, since that's how startups work. When you have limited resources, having a dedicated designer is generally going to be inefficient (unless design/UX are crucial to your particular startup in the early stages).

Also, non-designer founders have a lot of options regarding design -- they can outsource it, find/use a template, hack together some html/css, etc.