Based on my own research, I don't believe this information is correct. HDR10 video is encoded according to Rec 2100 [1], which states that the video is 10 bits and goes up to 10,000 nits. I know this to be true as I have written my own programs to encode HDR10 video. You are correct however that most HDR10 content is only encoded with a mastering luminance of 1000 nits [2]. According to this [3] forum post, all DV content (besides profile 9) is encoded using 10-bit HEVC stream. When people talk about 12-bit Dolby Vision, they are referring to Profile 7, which contains a full enhancement layer stored at a 1/4th the resolution, which contains the residual between the source and the Base Layer, allowing a player to decode the data to 12 bits. It appears [4], however, that most DV content only uses the minimal enhancement layer, which is only the dynamic metadata.

[1] - https://www.itu.int/dms_pubrec/itu-r/rec/bt/R-REC-BT.2100-2-...

[2] - https://www.rtings.com/tv/learn/hdr10-vs-dolby-vision

[3] - https://avdisco.com/t/demystifying-dolby-vision-profile-leve...

[4] - https://forum.blu-ray.com/showpost.php?p=16546620&postcount=...