Earlier quoted context omitted.
I don't think it's reasonable to assume that they're lying, but neither is it reasonable to assume that they're telling the truth. When a lot is at stake, we don't usually take people at their word--financial statements are audited, nuclear facilities are visited by international inspectors, and so on. I don't see why this should be different, but the Chinese government has continuously obstructed any such attempt. A…
Any evidence they have been misleading in their responses to direct questions about their samples would be a start. And you are making a claim about them lying, this comes unavoidably from saying that the VIW is the origin of SARS-CoV-2 as the VIW themselves are publicly claiming they have nothing closer than RaTG13.
I noted above that the WIV had a database of viral genomes. Public access to that database was removed in September 2019. They say this was due to repeated hacking attempts. They've taken no steps to restore access, or to make the database available in another format (e.g., a dump on a flash drive) that would clearly present zero information security risk. Do you believe their explanation?
And for emphasis, I don't think it's certain that they're lying (about RaTG13 being the closest known relative, at least; I can't see how anyone with the slightest domain knowledge would believe the "hacking" claim), just as I don't think it's certain that a company is lying about their financials when I want to see audited results. The point is that I don't know, and it's normal for people making an important claim to actively want transparency, to build confidence so people don't have to trust them. The WIV's behavior here is the opposite of that.