The 350 dB addresses the mathematical performance of the noise shaper, not the noise floor of the DAC.
Yes, such a figure represents the residual mathematical quatinzation error after digital math calculations are performed. Such figures can be made arbitrarily small simply by increasing the number of bits internal to some binary calculation. A number such as -350dB is only a marketing bullet point, and does not represent the measured noise floor of the DAC's analog output. It offers no practical benefit.
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