Is my HDMI audio out really an i2s connection?

No and furthermore no.

Don’t use the Toslink connection — you’re better off using SPDIF.  Upgrading your digital cable can do wonders depending on what you’re using now.  I was using a decent budget cable, but when I upgraded to this AZ MC2 cable (there are reviews online you can read) the sound improved dramatically — and I mean dramatically— in every way so that might be an avenue to explore, and your excellent DAC will certainly appreciate it.  Here’s one for sale just fyi…

https://www.usaudiomart.com/details/649951192-acoustic-zen-mc2-digital-coax-cable-1m/

Best of luck. 

Try it! Have fun. But... it is not i2s.

I run hdmi from my 1100 to a Moon 390 hdmi in and get 176k from SACDs.

I like the sound. 

Good luck.

I have a Sony UBP-X1100ES,  and I tried the hdmi marked audio out, into my McIntosh MDA200 dac hdmi input, and got nothing. I used a SACD which played fine using the coax.

I guess it's only for  input from a TV , but i don't see why it shouldn't work. 

One of the big issues with i2s is that they do not have standard pin protocol. If you do go with the dds route you need to make sure the pin out matches the pin in. Singer actually has pin toggles to match the dac i2s inputs of different dac. Guess  the main thing to know even though hdmi and i2s are the same cables they are not the same inputs or outputs.