It wasn't a 30dBi antenna for the control link, but a light moxon with 5.98dBi according to its datasheet. Standard 5.8GHz analog video, on the other hand, did require a ton of gain.
The amount of "db" to make a link is called the link budget. 20dbm (100mw) + 30db antenna is equivalent to over 300 WATTS out of a 3db standard 1/4 wave monopole antenna (assuming the rx side is equal).