Tibetan Vajrayana is often described from the outside as a religion of secrets, and the description is half right in a way that misleads. The preliminary practices are a good place to see where the line actually falls.
Ngondro is not secret. Its structure, the four thoughts, then refuge and prostrations, Vajrasattva purification, mandala offering and guru yoga, is published, taught in public courses, and printed in practice manuals a beginner can buy. The accumulations, famously a hundred thousand of each, are a matter of disciplined repetition rather than hidden knowledge.
What requires transmission is twofold, and it is worth distinguishing. First, even for an open practice a student normally receives the reading transmission (lung) and instruction from a teacher, so that the practice is done within a living relationship rather than from a book alone. This is a matter of how the tradition transmits, not of concealment. Second, the deity-yoga and completion-stage practices that ngondro prepares the ground for, visualising oneself as a deity, the practices worked with the subtle body, require formal empowerment (wang), and their detail is genuinely restricted: given only to those authorised, and not written down for general readers.
The atlas follows the tradition's own line here. It gives the general account of the preliminaries, because the tradition gives it too. It does not set out the restricted practices they lead toward, because to do so would be to publish what the tradition transmits only under empowerment, and an atlas that respected every other tradition's account of itself would have no warrant to make an exception of this one.