History is only immutable once it's committed to Fossil, and anything not committed is lost forever. This may sound banally obvious, but I think it's fundamentally the problem with Fossil's approach. With git I essentially commit my entire undo/redo stack. I end up discarding most of that later, while with Fossil I would never have committed those intermediate versions in the first place.

Without history rewriting, the rewriting simply takes place before committing instead of after.