It isn't mentioned because the author chose not to include it. Whether that's because this isn't meant to be a complete survey of the field and the author didn

It isn't mentioned because the author chose not to include it. Whether that's because this isn't meant to be a complete survey of the field and the author didn't think it fit the introductory nature of the piece, or because it didn't fit within the time limits of the talk this article is based on, or because Ruby doesn't do generational optimizations and the author saw no reason to wander down that path, isn't a question that expertise in gc algorithms gives much insight into.