Yes indeed it is currently as you say " Mainly for broadcast TV"
"Can't afford multiple streams for SDR and HDR"? Surely you mean broadcasters that have insufficient or limited bandwidth or in layman's terms so you understand Its a more convenient HDR solution because it negates the need to transmit dual bitstreams. I can dumb it down futher if that helps. Here is what the BBC say about it.
"In order to meet all of our requirements, HLG was designed to be a scene-referred system, just like conventional television. The signal represents relative light levels in the original scene, which allows pictures from a single mastering process or live production to be adapted to give the same artistic effect on brighter or darker screens at home. Only the display itself needs information about its own capabilities and environment to faithfully render the scene-referred signal, so metadata that describes the mastering display is not required. HLG also has native compatibility with standard dynamic range (SDR) television within the same colour format, which can be used for (UHD) SDR displays."
Youtube currently has HLG videos but i do not know yet if the xbox one x app or indeed if the Ps4 Pro app supports use of the HLG technology. i am still left with just guessing. I expect more data will surface shortly.
