AMD Support hasn´t answered yet via mail, so i googled again.
Techpowerup has reduced or updated the figures in their database - AMD Radeon RX 7900 XTX Specs | TechPowerUp GPU Database
The FP64 performance is theoretically just max. 1.919 TFLOPS with a "updated" 1:32 ratio regarding to the dual issue full FP32 61TFLOPS.
In real the FP64 will be ~0,9TFLOPS... (w/o dual issue).
Okay - the 7900 XTX is not so good for FP64... the RX6900/6950 will be better in that case.