A Sigma-Delta DAC would have everything in the digital domain that is required to generate a bitstream, followed by some way to convert the bitstream into an analog signal. There are many different ways to do it, though, because the Sigma-Delta principle is quite generic, and will work for analog and digital integrators as well as for discrete-time and continuous-time integrators. This may be why you feel that the block diagram is quite far away from an actual implementation.
Slainte!
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