Li-Meng Yan didn't blow the whistle in December 2019. She's a Hong Konger who started making crazy claims months after the outbreak began. She flew to the US, claimed she was persecuted, and now works for Steve Bannon. She's made all sorts of crazy claims, including that SARS-CoV-2 is a bioweapon.
The first pneumonia patients in Wuhan got their test results showing a suspected SARS-related coronavirus on 27 December 2019. The local authorities put out an alert on 30 December, which was instantly noticed by people around the world who track emerging infectious diseases. In other words, health professionals around the world knew about the Wuhan outbreak within 3 days of the first suspicious test results.
I don't think you realize the sheer volume of communication between people in China and the outside world. Many millions of everyday people are in regular contact across the border. There are many millions more people with VPNs who say whatever they want on Western social media. Even Chinese social media is way too active for the government to thoroughly monitor. Almost anything major that happens in China nowadays is known about outside China pretty much instantly. The idea that there's been a massive epidemic underway in China over the past year and that the government has been able to keep that information from leaking to the outside world is utterly implausible.
> They didn't even let a WHO investigation team into their country for almost a year
Which WHO investigation? The WHO visited on 20 January 2020, just three weeks after the outbreak was identified.[1] Months later, for political reasons, the Australian PM started demanding an "investigation." China didn't like the accusatory nature of the Australian demands, and it took months to negotiate a process that all sides considered reasonable. But the WHO was in Wuhan long before Australia began demanding a different sort of investigation.
1. https://www.who.int/china/news/detail/22-01-2020-field-visit...