Non-attachment as object of meditation
I think all these descriptions -  meditation on non-attachment, meditation with no goal, "just sitting", Diamond Samadhi etc. all refer to the type of meditation where your goal is to learn to be at peace with whatever happens in your phenomenal field - without either suppressing it or getting carried away by it.

One metaphor I heard to describe this, is the touch-and-go practice of the airplane pilots. You let the free-floating thoughts/emotions emerge and take shape just enough to get a flavor of each, but not to take over your awareness completely. You don't get attached to either the thoughtless state, or to any individual thought (or emotion) hence meditation on non-attachment.