[noggin]

My Akira Blu-ray includes a 192kHz 5.1 Dolby True HD track for instance. This will only be losslessly decoded on an ATV4K if it outputs 5.1 PCM at 192kHz.
(The Raspberry Pi is limited to 4.0 PCM at 192kHz - so it can't play this back losslessly - though it can handle 5.1 96kHz stuff)

Fair enough but my next question is how common are 192kHz 5.1 Dolby True HD Blu-rays in the real world ?

Or are we just going to argue over semantics ?     

 192k 5.1 is rare.  192k 2.0 is also rare.  96k is less rare (though not widespread) as it is used for some classical and concert releases.  

It's not semantics to point out that a statement is incorrect, and it may be important to you if you have those releases.

48k/24 bit True HD and DTS HD MA may well be losslessly decoded to PCM.  However 96k and 192k stuff may be downsampled non-losslessly if the ATV is limited to 48k/24bit output. Better to be correct than incorrect surely?

(The Pi outputs 96k 5.1 and 192k 4.0 PCM and people were careful to note that caveat when DTS HD MA and True HD decoding to multichannel PCM became available on it)