A TV is always going to be a TV as far as CEC device types go and Kodi has intentionally programmed Kodi such that it will only send CEC volume control if it sees an amplifier type device on the bus. As such unless Kodi changes this behavior it doesn’t really matter if any particular display supports accepting volume control or not as Kodi isn’t going to send volume messaging to it.
If your soundbar is connected to your TV then Kodi should send CEC volume commands to it, but if it is off then this may pose an issue where Kodi doesn’t switch its volume control scheme if the soundbar is actually still reporting itself on the CEC bus. What Kodi does as far as volume and keymapping, is that if there is no CEC amplifier type device (ie soundbar or AVR) Kodi will adjust PCM output volume internally. In this way one could have the TV volume turned up to whatever max volume they wanted and then a remote running through Kodi would then adjust the volume on the preamplifier side. If passthrough is enabled and you play a file that is being passed through (ie AC3, DTS, etc.) then the volume adjustment will not actually adjust anything as it can’t modify bitstream, only PCM. Since some setups can be a bit hard to understand on this topic it may be helpful to review the following guide that goes into some depth on various setups…