U-Plane latency is defined as one-way transmit time between a packet being available at the IP layer in the UE edge node and the availability of this packet at the IP layer in the EUTRAN node. U-Plane latency is relevant for the performance of many applications.
How did they drive it?
To calculate it 3gpp usually does hit and trial methods. Like they will tell you the standard things. In real time scenarios it will not be same.
For user plane delay they have sent 0 bytes payload data (IP packet) which has taken less than 5 ms delay approx 4 ms and further S1-U delay. So it came around 5 ms delay is the standard.