Voltage sag is quite common with electric vehicles. Reduced power output is possible when battery levels get low. Even Tesla cars will have reduced acceleration when battery levels are low.
This rider increased his risk of a crash in multiple areas:
1. Riding in Delirium
Delirium is the most aggressive ride mode and puts the largest demand for power on the board.
At 20% battery, I would have at least lowered down to Mission mode.
2. Riding Heavily on Pushback
Most likely this rider was riding on pushback for so long that he didn't realize the board was lifting. This is a common issue if you are not riding in a 50/50 weight position and have too much weight on the front of the board.
A good way to learn pushback is to switch to Sequoia mode, find a nicely paved area, and slowly build speed. You will hit pushback very fast due to the reduced ride speed. If you have the correct weight distribution on the board you will feel the nose lift easily. Just be sure to change it back to Mission after you are done testing.
3. Riding on Low battery levels:
I personally never push the board hard once I hit 20%. With voltage sag and other conditions that are out of my control, I feel more comfortable doing a gentle cruise at lower speeds until I can top off.