The tone curve does not follow ITU-R BT 2408, obviously.
The values in 2408 are just rough guidelines for live broadcasting.
The origin of the 203 nits comes from this:
Take an HLG live feed, take a typical Luma Waveform.
See that a typical Luma Waveform has a guide raster line at 75%.
Take this for reference white as this is convenient to spot on a Luma Waveform.
Then to translate this to PQ take the 1000nits HLG to PQ bridge and calculate were 75% HLG lands.
This will give you 203 nits.
The problem I have with 203 nits is that the precision of that number suggests that there is some scientific procedure behind coming up with that number (why would it be otherwise such a strange number)
But really what it should be called is “around 200nits”.
Then the 200 nits for graphics white are chosen for compatibility with SDR in simultaneous live broadcast of HLG.
In simultaneous broadcast you have the issue that the same HLG signal is shown on an HLG and 2.4 gamma Rec.2020 monitor.
If you acknowledge the fact that SDR 2020 monitors are typically set to 200-300 nits peak you need to make the diffuse white in the HLG signal brighter.
Otherwise the signal would look darker on the HDR monitors compared to the SDR monitors.
All of this is not applicable to motion picture and hence we did not follow ITU-R BT 2408.
I hope this helps.