Being created by a "mechanical process" from an existing creative work doesn't mean it's not a derivative work.

For instance, if I take a copy of $BIG_BUDGET_MOVIE, and resample the video frames from 1080p to 720p through a purely mechanical transformation, that doesn't make the output public domain.

It wasn’t written by a mechanical process, though. It was ported (translated) from an existing creative work into a new language.

Surely you wouldn’t say a Spanish translation of a Harry Potter work is in the public domain while the original work is under copyright?