TFT Central tested HDR400 True Black and HDR1000 in full detail here:
https://tftcentral.co.uk/articles/testing-hdr400-true-black-and-peak-1000-mode-brightness-on-new-oled-monitors
It turns out that... HDR400 TB produces a lower peaks at lower...
We know better than to trust manufacturer's numbers, but it's close enough at ~1200nits on Asus' monitors:
https://tftcentral.co.uk/reviews/asus-rog-swift-pg34wcdm
I mean, so far the roadmap has been true, and I remember doubters last year saying "DUDE, THERE'S NO WAY THEY'LL MAKE A 32" 4K/240HZ AND A 480HZ OLED, IT'S TOO EARLY MAN". And yet, here we are.