I can't speak for the author or to popular anti-China narratives in the U.S., but from a traditional geopolitical perspective China poses a threat because of territorial claims independent of Taiwan, most importantly its claims over the South China Sea. See
https://en.wikipedia.org/wiki/Spratly_Islands_dispute https://en.wikipedia.org/wiki/Territorial_disputes_in_the_So... Those claims pose a grave threat directly to global shipping and to regional fishing and mining rights, and indirectly to the largely peaceful maritime waters status quo globally.
Over the past decade China has also been making increasingly brazen land grabs along and near the Indian border. See https://en.wikipedia.org/wiki/Sino-Indian_border_dispute and, most recently, https://foreignpolicy.com/2021/05/07/china-bhutan-border-vil... Relative to the South China Sea that's slightly less of a concern geopolitically (ignoring the potential for another full blown war between India and China), but does evidence China's actual (not simply rhetorical) aggression.
Finally, North Korea is a protected state of China. The only reason North Korea has nuclear weapons is because China prevents military and, to a lesser degree, economic retaliation. China permitting North Korea to develop and possess nuclear weapons is a violation of China's role as a nuclear super power, especially in light of the proliferation risk North Korea poses. By contrast, the U.S. killed Taiwan's nuclear program the moment it was discovered, and strongly opposed both the Israeli and Pakistani programs up until they were already an accomplished fact. (Note that the U.S. had far more leverage with Taiwan--it could threaten to retract its defense umbrella, which would have passively permitted immediate Chinese invasion--whereas with Israel and Pakistan the only viable options would have required direct retaliation.)
Foreign policy and military analysts in the U.S. and, I would assume, China understand that the global economic tussle between the U.S. and China are distinct from the geopolitical security issues. (Strongly related, of course, but the calculus nonetheless significantly different.) The masses and certain political factions in each country don't tend to draw such distinctions, though.