Yes.The 5.1 mix is the lossy Dolby Digital 'core', which can be extracted from the lossless (7.1 Dolby True HD) Atmos stream!
It is called 'core' sometimes, by analogy with DTS HD-MA that really has a core DTS an then some 'extension' code that 'upgrades' the resolution from DTS to DTS HD-MA.
With TrueHD, (and Atmos) there are really separate substreams of TrueHD 7.1 and DD 5.1, within the same file, that are seen as different individual tracks, for extraction, by some tools.