Haiku is an amazing piece of work, but it would have saved everyone a lot of time and it’d be a lot further advanced if the original code was available, rather than having to implement it from scratch.
(Haiku developer here.) The correction is wrong; while some select portions of BeOS were released as open-source (most notably Tracker, Deskbar), the OS as a whole was not, and Haiku is thus largely a clean-room reimplementation.