OK, reporting back after some, eh, efforts....
I did buy this thing, even did buy two to send one to Amir...
https://www.amazon.com/dp/B09SD7Y5S5?psc=1&smid=A2I0EIL4I6JR4X&ref_=chk_typ_imgToDp
First things first... I wouldn't recommend buying one. It seems that it will only extract audio at 44.1K and 48K sample rate. No matter what settings you choose with the dip switches.
I found a tremendous rabbit hole of problems, trying to do what should be a simple thing, measuring stuff with REW and optical/digital interfaces. Probably not REW's fault, and perhaps quirks of my test gear, really... but man!! I have an old SB XFi-HD USB soundcard, it actually has TOSLINK in and out. Great! Loop back with optical cable, ridiculously low distortion and RTA numbers... Great! Plug in optical out to in on my Modi DAC, RCA's back to the XFi inputs, Great, measuring actual DAC performance! No problems, cant wait to measure the extractor.
I also did HDMI in from my laptop to the Onkyo AV amp... and saw Windows reporting rates up to 24 b/192k, 5 or6 Dolby and etc modes available, confirming that it's my RPi, not the Onkyo, that's the cause of my seeing only 8-16b from my Moode streamer box. Great!
And then, I ran into issues. This testing stuff isn't as easy as it ends up sounding in Amir's reports, I imagine there's a lot of background futzing that he has to deal with that we don't usually hear about. Upgraded to the latest beta REW, a windows update came in, some issues went away, etc... but still ended up with some weird incompatibilities of TOSLINK and such.. I'm not detailing many of the other problems during setting up I had that did eventually go away, like actual background noise during silence when using HDMI in on the AV, and so forth.
And, I get the HDMI extractor, the optical out seems to not be able to be seen by the XFi in. Won't sync, cant measure. Very frustrating. Also annoying that it seems to lose many of the video side HDMI abilities, and higher level Dolby multichannel features that windows was reporting the Onkyo capable of. My TV optical out seems also to not be able to be seen by the XFi in, though you can plug it into the Schiit Modi and it plays fine. Ah well. Was also hoping to try to measure the quality of the HDMI into TV -> TV optical out path. One thought of using the extractor, would be to replace the TV TOSLINK out, if it was "bad", like some reporting of lousy SINAD that some TV's seem to indicate.
I was also able, finally, to run TOSLINK out of the extractor, to the Modi, and get reasonable sounding sound out. Wasn't able to cross check it by using USB as source into the Modi and the same analog out into the XFi line in's, to try to compare quality, seems running two different USB audio devices is problematic for REW?
So, TLDR, this HDMI extractor does have both RCA Coaxial as well as TOSLINK, like I wanted, but seems seriously handicapped otherwise. My reason to have one was to add digital outs with at least most of the quality level of what I could play out. Yes, even those perhaps unhearably better hi-rez files and radio stations... If you don't want anything above 48k sample rate, or need all your multichannel options, this could be fine for you.