What I find the most interesting about the definition in SA 785 is that it makes the more subtle point that unless right samadhi is directed towards realizing the four noble truths, it leads to better rebirth in the heavens instead of liberation. In the context of thinking about dhyana relationship to samadhi, it isn’t mentioned specifically. But neither is any other specific meditation. Samadhi is presented as a general quality of an unconfused, focused mind.
So, from the point of view of the Sarvastivada (at least), right samadhi seems to be a catch-all for any practice that leads to a focused mind that can penetrate the four noble truths.
Admittedly, though, SA 785 reads very much like an abhidharma sutra. We know that later Sarvastivada abhidharma contains an elaborate model of liberation as a sequence of realizations of the four noble truths.
This is a good point. I think the Buddha’s decision to stop practicing the formless samadhis and switch to the dhyanas is in most (if not all) traditions. Still, what I wonder about is that it’s a four abodes of mindfulness that seem to get special treatment as the “one way to liberation” in a number of sutras. Perhaps that’s because it’s the gateway to more advanced meditation? Or was it considered sufficient by itself? I don’t know, just asking questions.