It says the acquisition time is = record length / sample rate.
The 5mS states is an example based on other example values and not a fixed time.
At 200 Million samples per second and using 1 MByte sample memory, the full memory represents a 5mS section of the waveform.
The "acquisition time" is the total duration of samples is can store at whatever sample rate is in use, for a given memory size.
If you were sampling at 100KHz with 1 MByte sample memory, the acquisition time would be ten seconds; that's the time taken to fill the display buffer.