According to Wikipedia, WeChat was first released in January 2011, while Snapchat only came out in September 2011. I conclude that Snapchat is more likely to be a clone of WeChat than the other way around.

Edit: To be clear, while I haven't used Snapchat, based on what I've heard about it I'd say that the two apps are nothing alike except for having some kind of chat functionality.

Thanks for the clarification. I read the "just" as "only" instead of "this week" and also thought it referred to WeChat instead of Bytedance's new Duoshan, which I totally hadn't heard of before.