The truth probably is that they know it originated (from animals) in China, and don't want the WHO to officially reach that conclusion. That's not particularly damaging to most countries, but in a communist country with tightly controlled information flow, the citizenry isn't shown a government making mistakes.
To be clear, these scenarios are entirely and spontaneously made up, but they're also entirely possible, and we've fallen into the trap of believing any old bullshit about China simply because it's possible, and reconciling our spreading it and our obvious ignorance with racist "inscrutable Chinese" tropes.
The reason we don't know more about China is not because they're quiet, the reason we don't know about China is because we ban and censor media coming from China (just as they do to us, but moreso.) Somehow it's become mainstream to believe that the public shouldn't know what the Chinese government thinks (or the Russian government, or the Iranian government) and that it's better to rely on rumor and retired generals.