That's incorrect. Wikipedia says that for digital audio via HDMI:
If an HDMI device has audio, it is required to implement the baseline format: stereo (uncompressed) PCM. Other formats are optional, with HDMI allowing up to 8 channels of uncompressed audio at sample sizes of 16 bits, 20 bits, or 24 bits, with sample rates of 32 kHz, 44.1 kHz, 48 kHz, 88.2 kHz, 96 kHz, 176.4 kHz, or 192 kHz
So maybe a future AppleTV4k will give us hires lossless music (AppleTV4k currently can only play normal lossless).
en.wikipedia.org