Now there's an official message from NVIDIA about this matter:
Late last year we updated G-SYNC ULTIMATE to include new display technologies such as OLED and edge-lit LCDs.Overclock3D argues NVIDIA made these changes for the good of the G-SYNC ecosystem. The site explains the certification hasn't been downgraded because peak brightness was never a good way to judge HDR performance to begin with. The updated standard allows for the inclusion of OLED-based panels with 600-700 nits -- and these panels are among the best in terms of HDR experience.
All G-SYNC Ultimate displays are powered by advanced NVIDIA G-SYNC processors to deliver a fantastic gaming experience including lifelike HDR, stunning contract, cinematic colour and ultra-low latency gameplay. While the original G-SYNC Ultimate displays were 1000 nits with FALD, the newest displays, like OLED, deliver infinite contrast with only 600-700 nits, and advanced multi-zone edge-lit displays offer remarkable contrast with 600-700 nits. G-SYNC Ultimate was never defined by nits alone nor did it require a VESA DisplayHDR1000 certification. Regular G-SYNC displays are also powered by NVIDIA G-SYNC processors as well.
The ACER X34 S monitor was erroneously listed as G-SYNC ULTIMATE on the NVIDIA web site. It should be listed as “G-SYNC” and the web page is being corrected.