Here is the thing with 0db idle, for it to be an actual useful feature, it really needs to be 0db "under light load". If the fans kick up the first time I do anything with the computer, then it isn't really working as intended. The point of 0db at idle is that you can do basic tasks with the computer and have zero noise. You can watch a movie, browse the web, write a paper in Word, etc. without the fans spinning up.
Yes, in the strict sense, when the 390 is at idle it uses almost the same amount of power as a 970, and the fans will turn off, even on a 2 slot card. However, you have to look at the power consumption numbers for other low load tasks. That is why W1z includes dual-monitor and blu-ray playback tests, both scenarios are low load tasks. Look at how much more power the 390 uses in those areas compared to the 970. The 970 is still down in the 10-12w range, and the 390 is up in the 75-80w range, which means the fans will stay off on the 970 but won't on the 390.
So, yes, in the strictest use of the term, the Gigabyte 2 Slot card does have 0db idle, it just isn't useful like the 970's 0db at idle. Because if it isn't active when you are actually using the computer, there is no real point to it.