Apple tv 4k second gen not hdmi 2.1

1. RGB is always 4:4:4. The other option, YCbCr, in this context can be 4:2:0 or 4:2:2 or 4:4:4. Which chroma setting you choose doesn’t matter all that much, as all streaming video is 4:2:0 in the source.
2. Do use “Ultra High Speed” HDMI cables rated 48 Gbps to enable all HDMI 2.1 features. Else the bandwidth (and features) will be throttled back to the specification of the lesser cable (5/10/18 Gbps).
3. Do verify the video settings on Apple TV, to see that the output format is as intended.

Verify that the LG settings on the HDMI port to which the Apple TV is connected, has “Deep Color” set to “4K”. Both are somewhat inaccurate terms, but that is how LG does it. (“Deep Color” set bandwidth limits, not color depth per se.) (“4K” is the ‘high’ setting for that, but Off also allows for 4K albeit 4K SDR@60Hz 4:2:0.) Connected (gaming) devices capable of up to 120Hz or VRR would assume high setting automatically, but Apple TV outputs a lower 60Hz at this time. On the TV, you can only change the settings of the HDMI-IN you are watching at the time. You can specify a different setting for each HDMI port.

⚙﹥(⋮)﹥General﹥Devices﹥HDMI Settings﹥HDMI Deep Color﹥

See page 104–105 of the LG User Guide.

From what I have read it's sampling it at 16bit and 48k and is very noticeable between the Xbox that can pass the sound directly to my preamp dac etc.

The audio from the streaming video is also 16 bit and 48 kHz in the source, so that is no reason for a difference per device.

What experience gave you that impression?

“256k” sound like a streaming bitrate (kbps) for a compressed encoded format. That is unrelated to audio bit depth or sample rate.