Can we do the 144-Hour Visa- Free Transit?

I am travelling from Seoul to Guangzhou on August 31, 2019. I will be in Guangzhou until August 3, 2019. We are leaving from Guangzhou airport to Hong Kong. Those this work for the 144 hour visa free transit??

We had tickets on the bullet train, but I have read that bullet train is not an option for the visa free transit. So we are planning on cancelling those. The other train is also an option, but I keep reading that if you do not have the tickets on hand, they will not let you in.