If Sai Baba's advice was just 'general ethical principles' and not deep Vedanta, how could it possibly lead to the 'highest benefit' for his followers? Wasn't it just superficial?

While it is true that Maharaj did not openly explain profound Vedanta subjects or give discourses on the Upanishads, his advice was primarily ethical. This approach, as described in Chapter 0, aligns with the Sadguru's practice of purifying the disciple's ground before sowing the seed of knowledge. This foundational guidance was crucial. However, the text clarifies that the advice received from Maharaj in the form of direct experience was 'infinitely more valuable than this open verbal advice.' It was this experiential guidance that brought the 'highest benefit' to devotees who approached him with faith, seamlessly turning their minds toward ultimate welfare even if they initially came with worldly desires.