I believe you're looking at the uDDC footnote under "External Master Clock Input" referring to *For USB and I²S inputs only.
That footnote is for sample rates, and is unrelated to the external clock input. See the * for sample rate...no * related to clock input:
High Sampling Rate Support
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PCM: Supports resolutions up to 768kHz*
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DSD: Handles formats up to DSD512*
External Master Clock Input
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Seamless integration with an external 10MHz master clock for precision synchronization.
*For USB and I²S inputs only.