Noise is one of the few things I'd be confident in saying MS will make sure is under control.
Yes, which is why we feel comfortable making estimates about the power of the console based on the intake vents.
There are basic limits to how well you can cool when aircooling things. Assuming that you are not doing anything stupid, to a first approximation the maximum cooling potential is linear to the amount of air you push through, while the amount of noise you make is linear to the amount of air
and the speed of the air.
This means that to provide the same amount of cooling, you have the options of moving air fast through a relatively small opening, which makes a lot of noise, or moving air slowly through a larger opening, producing less noise. There are ways you can fail and make more noise for less cooling if you make really bad choices, but there are no methods of magically pulling more cooling from the same airflow. The state of the art is simply too close to physical limits.
This is the reason why the second MS published the first picture of the XBSX, all the technically minded doubters started believing that 12TF can be real. Providing quiet cooling is a real challenge to something in a form factor of a more traditional console, while the upright box/funnel gives the option of putting a single 140mm (or maybe even bigger!) fan at the top, with a large, thin-finned heatsink directly below it. Add lots of openings somewhere at the bottom of the console and a relatively unobstructed path for air through it, and you have a perfect example of a system moving a lot of air through large opening at relatively low speed, allowing for both substantial cooling and low noise.
The reason the picture of the back of the console raises all kinds of questions is that the amount of vents for air intake seems smaller than ideal, especially since parts of it seem quite obstructed. Ideally, you'd want holes ~as large as the exhaust vent at the top of the console. This is why we're speculating that there are probably additional intakes somewhere. (Maybe around the perimeter at the very bottom?)