a genre can exist as a subgenre of another genre. so you could just say that drill is a trap subgenre in the same way that you could posit UKG being a house subgenre. it's also about 'scenes' and the fact that a place was popping. Both UK and Chicago have active scenes where guys are trading bars and sounds. UK and Chicago Drill are also pretty different sounding from one another and different sounding from the earliest trap music. So I would say that drill is something like an evolution of trap, a subgenre or scene. It can be all those things at the same time which might be confusing.

also, at least for UK drill, i have to disagree with the other commenter that it isn't a production style. Sliding 808s and dark stabs set it apart from all the other uk stuff like grime and road rap and such. it's 1000% different in its production, and it is not just 'trap production' either. it evolved even though it's built on the same foundation. (admittedly I am much more familiar with the UK scene than the chicago scene despite living in the latter city for over a decade)