It depends how far you want to take the definition of what you do away from 'traditional' PM. At a ~10 person company chances are there are a lot of things that need doing around interfacing with users/customers, identifying beta testers, following up, triaging product feedback, writing great docs and content, writing sample code and SDKs, etc. Sometimes one of the founders does it, but as the company grows, that kind of day-to-day is hard for a founder to stay on top of if they have other areas of focus too.
It depends a lot on the kind of project the company is building, of course. A developer platform vs a consumer app will have a vastly different need for such things. I work at a 10 person developer-focused startup and as an ex-PM with engineering chops, I jump between building the product itself, working on developer-facing stuff, and randomly helping out. I definitely wouldn't call myself a product manager at the moment, though -- if you really want that as your job title then you probably need a slightly larger company where you can take the reins on product from the CEO, while the CEO switches to focusing on developing the company itself.
I have also wondered the same thing in the startup context: is there a good role for me in a <10 person startup, <5 person startup?