The client receiving will connect to the one sharing the file. Routing the data through the IRC network would be unreasonable. This reveals your IP, which is kinda bad.
LoL... sharing your IP with a hacked server doesn't really affect me... but when I use magnets links from my home IP, I keep being harassed by Comcast.
Though I would indeed like direct file transfer for more usual purposes in other chat clients. Would be great with Matrix/Element.