For example, I just got a nice Ravpower battery (that was on sale for $20 before Amazon kicked Ravpower off of its store) (see here: https://www.ravpower.com/products/rp-pb172-portable-charger-20000mah) that extends my iPad Pro's battery life for at least an additional 6 hours (I haven't tested beyond 6 hours). No way I'm giving Apple $99 for this. $49 would be the highest price I'd say is a fair price; $99 is robbery.
Very different product. This product doesn't attach to the phone and stay there, and isn't very mobile.
Apple's product is a mobile, light-weight, cable free battery extender when you move around with your iPhone, e.g. being tourist in a big city, going on a long hiking trip etc.