This must not be "it", for this particular "magic" pouch, the sensor area is definitely covered. Pouch is a leather and it is black, plus some hardening stuff ( I guess some carton) between leather layers.
Besides, like I said earlier, I can cause phone's screen to go black (which is what must happen) with just a corner of a business card waived over the sensor. This piece of paper (thin carton) does not even have to be touching the sensor area (some 5 millimeters distance still does it). So, the light leaking should not be causing this.

What is currently fixing the problem is, that same business card inserted between the phone and the pouch wall. I guess I can just glue in a piece of carton in that area later on

I also tested the whole pouch for the sensor triggering functionality just by waiving different parts of it over the sensor, while there is an incoming call. Funny enough, most of the pouch's area does NOT make the screen go black... Not sure how is this all possible. I am now in process of "testing" other materials ))))

Sweet !!!