The redbook machine can't read a single layer SACD disc at
all. The redbook player can only read the redbook layer on
a hybrid disc, which is written in a language it understands.
Yup. That's it exactly.
IMO, some of the criticism of the redbook layer of CD/SACD Hybrid discs is due to the lack of attention paid to the redbook mastering as opposed to any inherent technical inferiority in the redbook layer of a CD/SACD disc. CD/SACD disc manufacturers devote most, if not all of their resources to the mastering/re-mastering and production of the SACD recording, and very little of their resources are devoted to the optimization of the redbook recording. This is especially apparent CD/SACD re-issues...like Pink Floyd's "Dark Side of the Moon".