Having specialization early on was a mistake. It was a difficult situation, and it wasn't entirely his fault that there was, for long stretches of time, little for him to do.
A startup cycle goes something like this: initial theory -> MVP -> validation of theory using MVP -> iterating to product-market fit -> scale
Even in that highly oversimplified flow, there are a lot of shifts from "product mode" to "development mode". Most of these steps must happen sequentially. If only half of the team is fully engaged in any given step, you get a lot of slack.
There's a joke that first startups should hire a bunch of developers to build the product. Then they should fire the developers and hire a bunch of salespeople to sell the product.
Which begs the question - why not just hire developers who can sell, and avoid paying all that unemployment?
Larger organizations can get away with having specialists onboard. Smaller organizations (2-3 people) can't afford this luxury.
Your advice would hold for a later stage startup, but it is unreasonable advice for an early stage startup.