[noggin]

It's 100% confirmed because that is what I get with my Yamaha AVR using an ATV 4K and the MrMC App. 

@wrxtasy isn't it only lossless for 48kHz content? Not sure it is for 96 or 192k stuff?    

I believe Yes for 48kHz audio content like TrueHD and DTSHD-MA.   

However True HD and DTS HD MA don't have to be 48kHz.

AIUI they can both also carry 96kHz and 192kHz lossless content - and this won't be decoded losslessly if the ATV 4K is limited to 48kHz PCM output - which is why I questioned DTS HD MA and Dolby True HD being losslessly decoded without a caveat (as you can't assume all DTS HD MA and Dolby True HD content is 48kHz)

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)