Not true. It even says on their website that the hard wire kit has build in monitoring and will switch off the dashcam once the car battery drops to 11.2V. The internal battery is just so it doesnt lose its settings. Its only 6 mins maximum, less the longer its parked. If you are relying on the internal battery for your parking mode then chances are you will miss whatever happened. .
If you read the instructions again (page 9 and 10) it's says several times to only fit it to a switched supply for normal use and to only use a permanent supply for surveillance purposes. That's the reason it has an auto shutdown and a parking mode. The parking mode only records for 30 seconds once activated and not triggered again and the internal battery when in good condition is good for 12-15 minutes use so has plenty of power for what it's intended for.