For example, adoption of GIMP has not likely materially increased over time (as a relative percent of total addressable market)
IMHO, what the author gets wrong what so many people who claim this (they are not the first, or the last) get wrong - open source as a whole sucks at building products, but is great at building infrastructure. (and pretty good at infrastructure products).
If you go look at areas where an open source thing is a de-facto market-leader, they are mostly infrastructure. Not all, but the ones that aren't have clear product management of some sort.
That is - the good open source products are often built/driven either by extremely product focused people (This is rare), or by companies driving them with PMs.
Not that the engineering doesn't matter - of course it does, but if you want a good and successful product, engineering is just a part of what is needed. Sometimes not even the main part!
Open source focuses mainly on the engineering part, often creating technical meritocracies that focus on making "The technically best software".
This is totally cool, but also totally orthogonal to building "a good product that people want to use".
Secondarily, you have the issue of survival in marketplace (IE There are a rash of OSS companies achieving near 100% of total addressable market share, and still going bankrupt due to inability to monetize). This, however, is not as big an issue as the first (as people can always still pick it all up when that happens)
Open source could always get better at products, but its historical development models were geared towards exactly the above (IE it does what it says on the tin)