For motion blur the shutter needs to be open longer but in auto mode it's trying to give you a crisp, clear shot based on the amount of light. ND filters act as sunglasses to reduce the amount of light so you can have the shutter open longer to obtain blur.
From your video, it looks like the bright sun was high in the sky. Perhaps near mid-day? Shooting earlier in the morning, or towards sundown or on a cloudy day will help reduce the light and produce better results. While you are experimenting and learning, let me suggest this. Get the drone in the air and while setting up your shoot, go to Pro mode and you will then be able to select your shutter speed. Keep selecting a slower and slower shutter speed until the shot begins to be overexposed and then you can back up a bit. During the day I have been able to get shutter speeds as slow as 1/3 of a second and produce nice results. After the sun goes down and you're able to set shutter speed at a full 2 seconds, that same area in your current video will really pop with light streaks of headlights and red trail lights.
Hope this helps.