I think it just says screen down because it will trip the proximity sensor and turn the screen off.
I was wondering the same thing because of how I use my phone in the car. I have a simple, magnetic holder in my dashboard vent that nicely holds my phone. I have an Apple silicon case and while the wireless charging does work with the case on, I have a piece of metal tucked between the case and the phone to give the magnet something to grab onto. And this piece of metal blocks the charging. I like my car holder so much I might give up on wireless charging altogether since I don’t want to have to remove my case to charge.