How does gravity make vents at the bottom useful? A normal non-passive laptop uses forced convection with fans, the natural convection should be completely negligible in that case.
Vents in the bottom just make sense, does it have to be explained? The fans aren't going to be running full blast during idle times or when portable for example. Saves power to use gravity.
Macs don't have to worry about that since they made a huge efficiency jump with M1. Before that they overheated due to poor thermals.
Arguably it helps so the air can go through the top holes in the keyboard better, you probably want openings on top and bottom for that.
But I still think this effect is negligible compared to the fan. I don’t think you would notice a temperature difference if you rotated your laptop 90 degrees with the keyboard vertical.