I don’t have a Android DragonBoard set up right now, so I haven’t tested that the instructions in the video are still valid, but I have a few ideas about things you can try to get it working.
The first thing I would check is the specifications on the LED. The signal levels on all of the 96Boards are 1.8V, this is much lower than the voltages from other maker boards (usually 3.3V, or 5.0V). The LED needs to be a red LED, most (but not all) red LEDs will turn on at least dimly with a 1.8V drive signal. Green or Blue LEDs usually need an extra volt or so to turn on (i.e approximately 2.8Volts) to turn on. What voltage battery did you use to test your LED? was it a 1.5V battery (an AA cell or similar), or was it a higher voltage battery (a 2.7V lithium Ion battery).
Other things to check:
Did you receive any error messages while entering the adb commands?
Did you connect to the correct pins on the connector?
Full Disclosure, I am an employee of Qualcomm Canada. Any opinions I may have expressed in this or any other post may not reflect the opinions of my employer.