Why are "premium" laptop vendors still putting vents on the bottom of their machines? Did they never try actually putting their laptop on their laps and realise how much that design sucks?
While it is possible to use a notebook on your lap, you are not supposed to. It is a terrible place that is unergonomic. You are supposed to put them on a table of some sort. If you are using it for more than something quick you should have a separate keyboard and mouse (ie attached to a docking station on your desk). The portable form factor is useful for meetings, presentations, or other such - but for real work they are terrible.
Since the lap what it was designed for, and a lap is a bad idea anyway: putting vents on the bottom isn't bad.
I also frequently use that computer, that doesn't have vents on the bottom, for watching movies in bed. And I don't need to think about vents. My last non-Apple computer was some ThinkPad, with vents on the bottom, and I remember always chasing some book to put under the computer to make the vents free. Boy, how I hated that.
People do all kinds of terrible things to their body. That you do something and seem to be getting away with it doesn't mean you should. Talk to a real doctor trained in this (your regular doctors probably is not) for details.
I've been living laptop life since 2002, IIRC. In all that time, the most stupid design decision I saw was on some HP EliteBook, where designers, in their infinite wisdom, put tiny legs on a laptop. Those four stupid pieces of plastic and rubber bite my legs every time when I try to use the convenience of having a mobile computer.
Fans on the bottom of the laptop case are firmly in the second place on my list. And no amount of ackchyually you're holding it wrong will change that.
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.
Hot air is lighter. Gravity help hot air go up, not down.