Transit Denied

Dear Sir/Madam,

Last week I was flying from Taipei (TPE) to Amsterdam (AMS).
I had a stopover in Guangzhou (CAN) of 7.5 hours. Due to delays, when I arrived at the immigration officer, there was about 6 hours and 45 minutes left. The immigration officer told me that the 72 hour free transit exemption is only for travelers who have a stopover of more than 7 hours. I cannot find information about a "minimum stopover time of 7 hours" anywhere. I feel like I have been wrongfully denied entry into China, is this correct?