Clean room design is a sufficient but not necessary condition to establish that something is non-derivative. It's possible that enough changes to the source make it non-derivative under copyright law.
As the Wikipedia link you gave says:
> Clean room design is usually employed as best practice, but not strictly required by law. In NEC Corp. v Intel Corp. (1990), NEC sought declaratory judgment against Intel's charges that NEC's engineers simply copied the microcode of the 8086 processor in their NEC V20 clone. A US judge ruled that while the early, internal revisions of NEC's microcode were indeed a copyright violation, the later one, which actually went into NEC's product, although derived from the former, were sufficiently different that they could be considered free of copyright violations.
As another example, BSD Unix evolved sufficiently away from AT&T Unix that only a couple of files needed to be replace to get away from AT&T's copyright.
Both derive from someone else's works, but that's a different sense of "derivative" than what copyright law cares about.
As the Wikipedia link you gave says:
> Clean room design is usually employed as best practice, but not strictly required by law. In NEC Corp. v Intel Corp. (1990), NEC sought declaratory judgment against Intel's charges that NEC's engineers simply copied the microcode of the 8086 processor in their NEC V20 clone. A US judge ruled that while the early, internal revisions of NEC's microcode were indeed a copyright violation, the later one, which actually went into NEC's product, although derived from the former, were sufficiently different that they could be considered free of copyright violations.
As another example, BSD Unix evolved sufficiently away from AT&T Unix that only a couple of files needed to be replace to get away from AT&T's copyright.
Both derive from someone else's works, but that's a different sense of "derivative" than what copyright law cares about.