Zander explains that all licenses are based on having copyright, whereas public domain means the lack of copyright. Therefore, one text cannot have copyright and not have copyright at the same time, making it impossible to dual-license something under both CC-BY-SA and public domain. Zander personally prefers copyleft licenses, so he is not bothered by the lack of public domain, but rather the lack of a good copyleft that can be used in BIPs.