His point, with which I still agree, is that running a business is different than starting a business.

More specifically, running a business with a known, stable business model is a very different activity than starting from scratch and discovering a new business. MBAs prepare you for the former, but generally not the latter.

A point that your history seems to match: you started and ran a business just fine without an MBA. And now that you have one, you've turned from entrepreneurship to working at large company.