They need to unionize. Writers in Hollywood get paid really well, because they have a union: The Screen Writers Guild.
It's not clear to me who this technical writers union would be negotiating with, or why they'd have leverage over that entity. O'Reilly obviously has a limited pool of people it can find to write a book about SVG, but that also implies those people should have leverage to command more than a $4000 advance without needing to unionize.
The nature of our industry and the widely disseminated information makes me really skeptical that could ever happen, even if there was a strong will to do so amongst developers and technical writers.
There are proven models for demanding higher pay for writing, but they require labor solidarity (and some measure of accountability from industry)