Do engineers working full time in a FOSS core team need someone to pay their salaries? Nothing the author said was against FOSS on a volunteer basis. What he said was it’s a challenging business model, unless you can reach escape velocity, and the window for that success is much narrower than simply selling a commercial product from day one.
There are real differences between FOSS that provides foundational capability (network layer, drivers, OS) and application layer projects. Firstly, they largely were built at a different time, secondly they were pure FOSS and not backed by a founding company that relied on it to keep core development going (Redhat’s business backed into Linux, so they benefitted from an existing core).
TLDR a founder who struggled to establish a business based on FOSS is saying don’t do it as a business because it’s harder than you think and can feel like a thankless endeavor.