I'm skeptical...
sceptical of??
Don't be. Diesel electric subs on batteries are notoriously hard to track because there is almost nothing putting out noise, the only way to passively track submarines. AIP subs are just as hard to track for the same reason, and add to that their underwater endurance is counted in weeks and not days. The sub that's quieter is the sub that wins, nuclear or not.
Now bear in mind this a technologically advanced sub from a highly industrialized country manned by a skilled, professional crew. And I cannot emphasize the quality of the crew enough, the best sub in the world is worthless if you put a bunch of amatures onboard it. Having Gotland on hand is an excellent idea for training, because if you can beat them then you have truly beaten the best at the business with the platform you're training against.
What to be skeptical of is when you see reports of (Insert Third World Country Here)'s air force in an exercise kicking the butts of American F-15s and F-16s, like in India a few months back.....those exercises are basically rigged, the US forces have their hands tied behind their backs - serves two purposes -1) The local third world country gets confidence and favorable pub 2) The Air Force, which is notoriously good at getting money, gets more ammo to ask for LOTS of F-22s, which is their main procurement goal.
In THIS case, however - believe it - a well-trained modern sub like this Swedish one is a very, very, very difficult opponent.
Really all modern subs from any country with a modicum of training is incredibly dangerous.
There actually were no "submarines" in World War II, other than the very late war German Electroboats, hydrogen peroxide subs, etc. - they were all "submersibles" - spent the vast majority of time on the surface.
Other than one shot in the Falklands, we've never really seen what subs can do now - and they've advanced faster than antisubmarine warfare advanced. In any large-scale naval war I suspect modern well-trained subs of any kind would rule the seas.
Fortunately, few of our potential opponents are likely as well-trained as the crew of the Gotland. It's very smart for the US Navy to lease those guys to practice agains (and I have a feeling the average Swedish seaman isn't violently protesting being stationed in San Diego - I wonder if they brought over their familes and put them up in San Diego, though.)
I'm not, diesel electrics have been pulling off the sneaky bastard routine during NATO exercises for decades. I talked with a dutch sub crew in the late 80's and they had finished a recent exercise where they nailed one of our carriers.