If the Lillium is as safe as a AB 350, why not just certify to all the existing EASA/FAA Part 25 and 29 regulations?
For one, none of the powered-lift aircraft are considered transport category so it would have been Part 23 and Part 27. Unfortunately to use those regulatory parts for an eVTOL or AW609 certification would require an excessive amount of exemptions and other work arounds.
The FAA had used the 23/27 route initially but ran into issues so they made a right turn and elected to follow the EASA lead and write a new regulatory Part for powered-lift. However, to keep things moving until a new part can be written, the 609, Joby, Archer, etc aircraft are being certified as a Part 21.17 Special Class aircraft which cherry-picks various individual regulations from various certification Parts.