This is how I see it...
HDR on Android TV before Nougat has been proprietary. Apps supporting adaptive streaming like Netflix or Amazon Video were given the possibility to determine HDR capabilities via some private non-standard API. In case of Kodi there was/is no such check. You just pass a stream to the video decoder and it either plays (TV supports the HDR video format/metadata) or fails.
Internet streaming is adaptive. The player decides which stream to play based on the available bandwidth and device capabilities. That is why Nougat introduced some HDR APIs to determine those capabilities in a standardized way, see here. Sony/MediaTek do not implement those in their Nougat firmware which is most probably the reason why certain apps like BBC and YouTube do not play the HDR streams but instead revert to the SDR pendants. That's the price you pay for chosing a cheap and unsupported SoC like MediaTek.
