Since most cryptocurrency blockchains are fundamentally alike, the ability to add arbitrary data is not exclusive to bitcoin alone. (cryptotimes.news)
YOUR quote: «it is not a database to store arbitrary data or it would end up as some kind of massive database that must be downloaded by every node.» (themerkle.com)
ExtraData: An array of arbitrary data which can be associated with each maplist entry; mainly used by the game at runtime (thatvideogameblog.com)