Apple Music Hi-Fi has two quality settings. Apple Music Lossless and Hi-Res Lossless. Compared to standard lossless audio (CD quality), Hi-Res lossless audio is 24-bit with a sample rate of 192kHz, while standard lossless audio is 24-bit at 48kHz. more simply put, Hi-Res lossless audio is seen as a better version of Apple Music lossless audio.
To solve your problem, you can use AudFree Apple Music Downloader to get Apple Hi-Res music for playback on your Apple TV.