Yes, the detach has to be sent manually, it's something the user initiates.

I kind of like to keep the channel open on PART as well so I can make the final choice to close the tab manually, detach keeps the channel open in the ZNC but you don't see it in the list so it's not a technical PART, it's usually an "important" chan that you want to keep and way want to take a look at the chats or links even if you click detach too quickly. Anyway, I'd keep them all open on PART even just not from the ZNC, an extra option to check would be great.