That is nonsense. Plenty of folks took it seriously, their pandemic responses teams activated early January. China shut down whole provinces and tanking their economy and risking unrest. That alone was a strong signal (and WHO raised their warning levels at the same time). I mean seriously, what would have been a warning sign if not that? Obviously China (as well as other countries hit by SARS) took it seriously at the latest toward the end of January (again, there was an unprecedented shutdown). Other countries, including US and Europe did not react until late February to March. Folks such as the RKI in Germany have acknowledged that they should have done more contact tracing, for example.
In addition it is a weird cover up that managed to create perfectly predictive data. So far transmission rates, lethality, effects of age and comorbidity all seem to line up with data all over the world so either they new the correct numbers and fudged it to much or they covered up something that had no impact on our undersanding. In that case if something was covered up, what would it even theoretically be? Everything that happened in China is now happening elsewhere with the same intensity (or worse, depending on the response). So what is being covered up that so well matches all observations so far? I do not expect the Chinese government to be forthright, but it is silly to assume that this causing in poor responses elsewhere as, again everything that came out of China so far was shown to be fairly predictive. In addition to all that, there was also data coming out from countries that started early testing, so by February we also had range of strategies on display, including total lockdown (China), enhanced contact tracing eventually followed by testing (e.g. Taiwan, Singapore, Vietnam, South Korea). Well, or nothing.