For what it’s worth, the author (eventually) explains that by “AI doomer” they’re not talking about people who are generally skeptical or pessimistic about AI’s influence on society, but rather people who believe that AI will literally kill all humans:

“[T]here are plenty of people who do believe that AI either will or might kill all of humanity, and they take this idea very seriously. They don’t just think “AI could take our jobs” or “AI could accidentally cause a big disaster” or “AI will be bad for the environment/capitalism/copyright/etc”. They think that AI is advancing so fast that pretty soon we’re going to create a godlike artificial intelligence which will really, truly kill every single human on the planet in service of some inscrutable AI goal. […] They would likely call themselves something like ‘AI Safety Advocates’. A less flattering and more accurate name would be ‘AI Doomers’.”