It's not really a surprise to me, or anyone in the submarine community, that Russian boats are becoming quieter. So are ours. I dont think we are losing ground to the Russians, or anyone else for that matter.
New tech means quieter boats. I remember when I was on my last boat, (USS Salt Lake City) the Seawolf had just been launched. Seawolf was said to be as quiet at a flank bell (full throttle, for you non-submariners) as we were tied to the pier. The Seawolf class was, at the time, ground breaking tech. THE most capable, and the quietest nuclear powered submarine in the world. Seawolf was expensive, so we only built 3. The follow on class is the Virginia class boats, even better, every bit as capable, even quieter, but less expensive. I have been aboard one...absolutely incredible machine. Designed for littoral warfare, specifically for detecting and killing high end diesel boats in shallow waters...where diesel boats are the most deadly.
This new Russian boat is apparently non-nuclear powered. Russian diesel boats were always pretty good, but very range limited. This boat sounds like it MAY even have Air Independent Propulsion (AIP), so that she may recharge her batteries without snorkeling. Most likely improved battery tech, so she can run longer and faster between battery charges. Sounds like she is cruise missle capabile, so I imagine she has some form of vertical launch system. Sounds like it may be an interesting new boat our Russian friends have there!
Quiet as she may be, no worries, there are various ways to detect and track submarines. We have become quite astute at that, using acoustic, magnetic, and 'other' means.
One more thing...as far as the submarine force is concerned, the Cold War never ended. We still keep close tabs on the Russian boats...just like the 'good old days'.