You can never believe anything that Chinese say. That's their nature. Past experiences in similar situations have already proven this.
The Chinese released the full genome of this new virus to the researchers in record time. The US did a full sequencing of the virus from their first patient and this proved to be an exact match with the data released by the Chinese.
They might have this already in the Wuhan lab, and got accidentally released. Who knows...There were several leaks before with SARS.
Also they have said that the first Chinese patient never visited the Wuhan market, though it later spread from there.
The answers to these questions depend on knowing how much the Chinese government is hiding.