Surround Sound vs Dolby Atmos vs Spatial Audio

[struts]

A post by @Shulaf in another thread made me realise that I really didn't understand what Apple's Spatial Audio is or how it relates to Surround Sound and in particular Dolby Atmos, furthermore whether Dione supports Spatial Audio and what on earth SPACE™ is and how that plays into it.  After a bit of digging and some background reading (these two articles are good primers) I think I am starting to get a grip on it but there are some aspects I haven't been able to make sense of.  Hoping folks here with a better understanding can chime in and enlighten me. 

So, as I understand it..

• Surround Sound is where the sound is mixed into multiple channels which fire at you from different directions creating the impression that you are in the middle of the action rather than that everything is taking place in front of you.  Some systems simulate surround sound by reflecting sound off room boundaries like the side and back walls to create that impression.  Dolby Atmos takes this a step further by allowing content creators to define sound sources as "objects" in 3D space which can move about, the Atmos technology then converting this into a mix for the number and placement of the available channels.
  
• Spatial Audio is a technology that is separate from but compatible with surround sound that uses gyroscopes and accelerometers to measure head movement and fix the perceived direction and strength of sounds in space as opposed to fixing them relative to the device that is creating them.  So for instance if you are listening via headphones and you turn your head to the left the channel balance is altered so that a sound that was coming from the left now appears to be coming from straight ahead.  Also, as you move around the system will remix so that you feel like you are moving closer to and further away from various sound sources.  This makes sense to me in a headphone listening context, where the speakers are fixed relative to your head but not relative to real space, and I can understand how it would work with devices that contain gyroscopes and accelerometers that can detect and measure head movement.  
  

What I don't get is:

• What relevance if any does this have in scenarios where the speakers are fixed in space?  When you turn your head or move around your head moves relative to the speakers in reality without needing any dynamic remixing to simulate it.
  
• How can this work technically in devices like Dione that don't contain gyroscopes or accelerometers so can't track head movement or orientation changes?  
  

I think the answers are "none" and "it doesn't", so while it supports Dolby Atmos as far as I can see Dione does not support Spatial Audio even when playing an Airplay 2 stream.  

To add some further spice Devialet talks about "SPACE™ technology": 

SPACE™ technology opens up a new dimension of sound. A patented algorithm upscales any mono or stereo signal into a 5.1.2 signal for an all-enveloping, muli-layered, deeply immersive sound experience, whatever the content you’re enjoying. 

But what on earth does that mean?   Without any metadata how can you make 5.1.2 out of a mono signal?  "Man-guessing"??  I was unable to find any patent filing that might explain this and also notice that while I was writing this post Devialet's website appears to have been updated, "patented" being replaced with "proprietary" in the above quoted text which I copy-and-pasted directly from the page but is now sunk without trace.

If anyone here can shed any light I'd be fascinated to understand if this is technical innovation or simply marketing innovation.