Those lights have a directional lens that creates a wide beam horizontally but very narrow vertically. If you mount it vertically than it'll appear much dimmer when viewed by someone who's not exactly behind you.
I can see they discuss that effect in the marketing materials. In reality when you look at it I don't think it is at all noticeable
Reality:
@lemonade started
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Those lights have a directional lens that creates a wide beam horizontally but very narrow vertically. If you mount it vertically than it'll appear much dimmer when viewed by someone who's not exactly behind you.