Can someone explain the new hills please

[Imp]

No change to LOS as far as I have noticed, being behind a ridge line is not safe the game calculates LOS as stated above. The further away the "deeper" you can see.
Easy to see in practice by deploying units on & behind a ridge line & looking at their LOS.
The further back you are the more nearby hexes are obscured but you can still see the far ones.

As to the "if I can see you then you can see me"
If vision is marginal due to intervening hindrance terrain & dust one more shot can block LOS due to the invisible smoke produced.
On very rare & occasions I think it is possible for one unit to have LOS while the other doesn't. My guess hindrance calculation varies by a tiny amount due to crossing a hex side or some such & if vision is only just possible it blocks one.
Its so rare its not worth bothering about & there are so many factors involved I could be wrong, it might just be one units higher experience making the difference for example.