This could just be a coincidence. There are multiple people in the thread claiming they set their password to the same thing and have suffered no consequences.
Don't spoil the party. It is much more fun to pretend that WeChat stores a billion passwords in plain text just so some bureaucrat from the CCP can check if there is anything offensive there.
you don't need to store it in plaintext, this was probably caught by something general like the great firewall or golden shield then traced back to her wechat account