You're not drawing the right conclusions from the data presented here, partly because you may not understand how clocking schemes typically work in audio hardware. For the most part, designers don't do any type of reclocking/de-jittering on I2S data, whereas it's relatively straightforward to add an additional clock recovery PLL/reclocking/do ASRC on S/PDIF data. You can see in Amir's original post that the I2S interface leads to additional jitter-related sidebands, in part because of this.
I'd rephrase your conclusions as follows:
- In most conventional designs, S/PDIF permits for lower-jitter clocking in an external DAC than I2S.
- For an external DAC with poor/degraded clock recovery, there are very limited practical scenarios where there would be a benefit to an I2S interface (this would require very short cables, proper electrical clock distribution, and a clean clock master). In most actual scenarios, like the measurements Amir showed, the I2S interface is still inferior.
- For external DACs, the I2S interface can never be as good as a well-engineered S/PDIF interface. At best, under perfect conditions, I2S can be just as good, but the engineering pitfalls are significant.
