Gravity Boots pull up bar help

Hi guys

I've got a pair of gravity boots and I was looking for a pull up bar to just screw into the doorway. However, the only pull up bar at my local store had a disclaimer on it stating "not suitable for exercising upside" which freaked me a little. What kind of bar do you think would be suitable for exercising upside dow?
Personally, I wouldn't trust doorway bars for hanging upside down. Find bars outside in parks etc (and gyms after corona) and hang there maybe?