That part is right. HDMI carries it but HDCP steps in
Along with other rules. You may want to consider unplugging all HDMI and removing power since we have seen HDCP handshake failures that only seem to go away on a full power cycle. I'm going to shortchange you here since HDCP handshake discussions and web pages abound.
OK, so the normal chain is from SOURCE to HT and then on to the HDTV. The optical is put into play when the HDTV is the source such as OTA (over the air) or things like game consoles that you didn't connect to the HT first.
However pass-through is sadly hampered or hobbled by HDCP rules. Let me be clear there are no technical reasons all this can't work. It's all about the studio's worry that high definition content might leak. We only need to google "plugging the analog hole" to see prior content about all this.
This is why I consider HDMI and BluRay to be nice but can be consumer unfriendly.