Why was a clean-room approach necessary here? The UIUC License used by UBSan is extremely permissive, so going through extra effort to avoid creating a derivative work doesn't make much sense to me.
- The author wanted the personal challenge for the task
- The author wanted the code licensed under a BSD 2-clause and felt the existing license didn't fit her/his ideals.
Of course, I could just be misunderstanding, and they could be using "clean-room" as a synonym for "from scratch", rather than the meaning I linked to.