You're right.
I know some config missing somewhere. The TV is not able to sense the Atmos compatible AVR on its ARC port?
I'm watching Our Planet and some other Atmos content from Netflix, some test DolbyVision + Atmos content from Dolby Access app, TV shows only 'Dolby Vision' on the top right when those from Dolby Access is played. I know certain titles are Dolby Vision + Dolby Atmos on Netfliz, those are shown only Dolby Vision, that means its not sensing the Dolby Atmos capbilities on AVR for some reason!
Tried setting it to TV internal speakers, with Dolby Atmos set to ON and I see both Dolby Vision + Dolby Atmos on the top right corner when these test content and Netflix content are played.
Also, tried following couple things with not much luck,
1> Sent the TV Audio to AVR using Optical, just to see if I can see Atmos enabled on content (that is actually atmos enabled)
2> reset both the devices (LG C8 and Marantz sr6013) to factory settings to see if I can get atmos working over hdmi, looks like either I'm repeating the mistake excellently or some configuration I'm missing.