I think you have a bit of an idealized perception of the Great Firewall if you think it allows controlling all communication. That's certainly the goal, but in practice, the implementation is far from achieving it.

Well-known historical events like the Tiananmen Square protests can be censored because they're so well known, so the censors know what kind of content to block. (But people who want to talk about them anyway also know which keywords to avoid to evade censorship. It's a cat-and-mouse game.)

New scandals become known all the time and cannot be censored until the news has spread far enough to reach the censors. For example, when Li Wenliang warned his friends about SARS infections in his hospital, the censors didn't know they had to censor screenshots of that conversation or his name until it was too late. When Ai Fen gave an interview to Renwu, the censors didn't know they had to censor it until it was too late.

In the end, the Chinese government had to honor Li Wenliang as a martyr and Ai Fen as a hero to somewhat calm the waves of public opinion. It's extremely unlikely that they'd be able to successfully suppress the information if the virus was lab-made and someone blew the whistle on that.