Bram Cohen developed BitTorrent and released a (Python) reference implementation in the public domain (later under MIT and then GPL licenses); he later founded BitTorrent Inc. and assigned this implementation to the company to maintain. Eventually BitTorrent Inc dropped this codebase altogether and became closed-source with a completely separate project.

Libtorrent is just one of many independent BT libraries that have been developed since Bram published the first BT client.