Pre-PMF startups typically have no dedicated product managers — the product co-founder fills that role directly, immersed in constant customer contact, every deal won or lost, and every experiment run. This immersion is the source of the founder's Product Sense (see also "Founder Mode" Misread as License to Micromanage — the Real Lesson Is Founder-Style Leadership).
This explains two downstream facts: why VCs like Y Combinator require or actively matchmake a product co-founder for startups without one (compare VC Seed-Funding Model vs. Corporate Project Model), and why startups have a natural size ceiling — the model only works as long as one person's personal involvement can span the whole product.
Once a company reaches PMF and needs to run multiple product teams in parallel (e.g., 8 product teams), the founder's personal-PM approach stops scaling, and the company must develop the genuinely new skill of enabling other people to develop and exercise product sense — the shift that founder-style leadership addresses.