A civ won't list all of it's techs in the trade window. This is fairly easy to see when you have reached a high level of espionage with them, and compare what you find with what is offered in the trading window. So, it is very possible that the Drengin possess those techs, and just aren't showing them. Then again, who knows, it could very well be a bug.
Yeah I never heard of them not showing thier techs in the trade window, after all I can't hide my techs from them as long as the trading tech options is on in the game. If tech trading is off then I can't see thier techs. This game tech trading is on and I have traded plenty of techs already. Also I've noticed when in a game with high espionage level that when I steel a tech it is one that is displayed on thier list so I don't know what you are talking about sorry. Please help me to understand.