If you mean Tidal, then you are correct that Hifi (CD quality) is the maximum. This is not a limitation of the Naim app but because Naim does not license MQA, which is the only way you can get higher than CD quality from Tidal. (Details about MQA were mentioned by Mr. M above).
Airplay is limited to 16/44 anyways.
With a Qobuz subscription, if it is available in your country, which streams “real” hi-res as FLAC files, you can play hi-res up to 24/192 with the Naim app. There is no need to use Chromecast in this case. In fact, using the Naim app is recommended over Chromecast for sound quality. If it matters on the Atom is another question, but you can compare yourself by using Chromecast from the Qobuz app vs. using Qobuz in the Naim app. Compare this post by Naim’s Software Director: