The ocean is big and sonar isn't a secret. Much like with radar and figuring out that Russia's stealth planes aren't stealthy, chad sonar enjoyers can figure out the shadow of a submarine. Russia uses it's shit for propaganda so everyone knows it's not as good since we've been seeing it, and more importantly hearing it, consistently for decades. Same with every other nation worth a damn. Again, only China is kinda iffy but considering it's China, they can produce subs sure, but the quality is chinese quality. Same as their aircraft carrier (considerably more important to their expansionist goals) being shitty. Japan is pretty adept at chasing them down so that says something.
Also nuclear doesn't really factor into this. A stealth submarine is judged off of it's stealth, not it's powerpack. Nuclear trades reach against the quiet of pure battery. It doesn't even matter if diesel-electric is cheaper or nuclear submarines are the big-dicked wonders of the world, the appropriate physics and basic ideas don't change. We know the general material they're soundproofed with, we know their size, shape, and propellers, and there's plenty of nerds who've done the math and figured out that their video games are overstating Redfor submarines and understating American subs for balance.
Submarine tl;dr 2 diesel-electric boogaloo: I'm not being hyperbolic or chanting USA to say that the US is not balanced and set the standard insanely high.