HDMI-CEC is what it is. But, figuring out what it is and how to turn it on or off can be challenging with manufacturers all having different designations for it.
Even though eARC can be used without HDMI-CEC on in the TV and AVR, that does not mean a product by a particular manufacturer has been made to work that way. Use a TV that supports eARC with an AVR that only supports ARC and HDMI-CEC must be turned ON for any audio signal to return to the AVR.
LG TVs will turn Simplink(HDMI-CEC) ON automatically even after one has manually turned it OFF if it recognizes HDMI-CEC equipped devices connected to HDMI #2 especially.
If somebody is using an LG TV and AVR that both support eARC, they can turn HDMI-CEC OFF in their AVR and every other device connected to it and still get audio signals to the receiver using eARC and the TV input source will have to be manually selected as the AVR source input will now not change automatically.
When using an AVR that supports ARC but NOT eARC with any eARC equipped TV, one must turn ON HDMI-CEC to send audio signals from the TV to the AVR. To keep other devices from turning the TV and/or AVR ON or OFF when using HDMI-CEC for ARC, one must make sure HDMI-CEC is turned OFF in all of their other devices.
Turning HDMI-CEC ON or OFF in various devices can be like a going on an Easter Egg hunt as different device manufacturers have different designations for it. Those with LG products will need to look for Simplink, those with Panasonic products will need to look for HDAVI, EZ-Sync or even VIERA Link. Those Samsung products will need to look for Anynet+ and Sony folk will need to look for BRAVIA Sync.
The Apple TV 4K works beautifully with my LG TV and Onkyo receiver using HDMI-CEC. Though, it and other devices like the Roku Ultra have quirky EDID characteristics that can really screw up some setups.
For those who do not want anything to do with HDMI-CEC, The Apple TV 4K can still adjust the volume level of an AVR using IR. The Apple TV 4K also has a Learning function that I really liked using with my Denon Universal Remote Controllers. I would assign a source input button on the remote controller to use with the Apple TV 4K and then assign corresponding buttons by holding them down and watching the learning progress bar on the TV. Once done, I didn’t need the Apple TV remote controller for anything more than typical use.
So, if one does not want HDMI-CEC functionality ON at all in the Apple TV 4K, go to Settings>Remotes and Devices>Control TVs and Receivers and set it to OFF. Then, if you want to adjust AVR volume with Apple Siri remote controller, go to Volume Control and set it to Receiver via IR.
Anthem provides a nice explanation and implementation of HDMI-CEC. But, not setup correctly in the Anthem as well as in every other device, all bets are off.