I read this on AVSforum. seems the Roku spec is not complete.
"As far as Dolby Atmos, the 4801 isn't certified and lacks the decoder, but it still will pass Atmos through to the AVR. That's a little hidden secret Roku doesn't like to talk about (same goes for the Streaming Stick)."
This explains why I see Atmos on my receiver using passthrough, although the roku documentation seems to say the player doesn't support Atmos.