To give two sentences of background, I've been running businesses for 13 years, and running a startup for 4 years. The first business I built by myself from nothing, and the second I built alongside my technical co-founder.
Firstly I'm concerned that you seem to think that I'm suggesting finding a 'specialist' business co-founder. Being a co-founder is the exact opposite of specialisation. If you're a co-founder period, you do ANYTHING it takes to move things forward, no exceptions, ifs, ands or buts.
Also, without meaning to be picky, I believed you identified your co-founder as terrible. Someone who, as you described, believes that doing 'thought experiments' and trying to get twitter followers is the appropriate thing that a business co-founder should be doing in an early stage startup, is objectively a terrible business co-founder.
If your business co-founder is doing his job properly, there should never be any slack. Exactly the same as a technical co-founder. Companies, especially early stage companies, can and must be doing product mode and develop mode simultaneously.
I've never heard that joke, but as you grow your company and get more customers on board, who have more requirements, it's pretty important to have both salespeople and developers working in tandem to build out a company.
I agree with you that larger organisations can afford having specialists and smaller organisations cannot.