This is contrasted to denialism which denies claims out of hand. This is not based on scientific data, but gut feeling or motivated reasoning.
Dr. Ichikawa is thus describing denialism in his tweets.
* https://en.wikipedia.org/wiki/Skeptical_movement
* https://en.wikipedia.org/wiki/Denialism
edit: Here is an article on the distinction from one of the strongest figures in the skeptical movement, Steven Novella. It even focuses on the topic of climate change.
https://theness.com/neurologicablog/index.php/skeptic-vs-den...
However the colloquial, general meaning is still that of scientific skepticism. That's what comes up when you search skepticism on Wikipedia, reddit, or various definitions. That's also what is reasonably being referred to in a Hacker News post; a community that often subscribes to the same values expressed in skepticism.
Unless you know this professor and can put his words in their appropriate context, then the whole thing reads as decrying an entire movement of scientifically-minded people.