I don't think this is really any different than Free Software developers writing programs for other closed platforms like Android, iOS, macOS, Windows, etc.
Sure, it's probably better for the users to have a free software alternative they can use, but in the end writing free software for these platforms just ends up adding more value to the platform, and reducing the incentive for the user to switch to a more free software platform like GNU/Linux.
The other message I'm picking up from the author is that free software developers are not as interested in web technologies as they should be. I'm not sure that this is true. There are a large number of javascript projects on Github, for example. Also there's quite a large amount of free software projects covering the backend of web development as well. Really the problem is hosting the service, when there's no money involved. For this, I think some kind of distributed effort is key, each user "pays" for the service by helping to serve it to other users.
Maybe the author is speaking about just user-facing applications, not frameworks, and infrastructure projects? This is also a problem with free software desktop applications, not just the web.
I guess I would be considered a "free software fetishist" by the author. I like C, I like vi, I like Linux, I like POSIX, I like CLI, and I don't really like the web that much, especially for applications. And I don't really see anything wrong with that. Let the people who are interested in the web work on it, and I'll work on what I'm interested in.