In this situation, it meets the requirement b), which the beacon effect’s length is longer. In the video you can learn that the beacon effect did overwrite the potion effect (the length refreshed to 16s)
Still in 1.13. I think he problem is that while you're climbing a ladder, the game thinks you're in midair so it will allow you to use the elytra. A related problem is that if you're going down a ladder and hit the space bar twice, you'll start gliding with your elytra and eventually crash into the ground since you can't hang on to ladders while gliding.
Can confirm for 1.13.1. Just tried and got kicked for a few times.