They were just stopping in China, supposedly, on their way to European destinations. You generally don’t need entry documents for a country if you’re just going to be in a transfer lounge and never leave it and actually enter that country. I’m sure the terrorists (if that’s what they were) knew this.
Amsterdam via KLM was the supposed destination. I remembered it as I have done that route myself.
Head guy for Malaysia Airlines said all of them had visas and China had the info.