My bad - I noticed that the screen becomes brighter than it is possible to manually set it to when it's in harsh sunlight, so I assumed that this was a result of it increasing the SDR brightness limit past 500 nits, I didn't know that such conditions were required merely to push it to 500 nits. Thanks for the info!
The SDR nits never go over 500 in my experience. But it's possible I haven't tested this correctly.