Granted, the game's AI routines, animations, and physics are basic to say the least, but the sheer insane amount of active NPCs it swarms the screen with is something that "just three Wii CPUs taped together" certainly couldn't pull off.

agreed the wiiU is definitely stronger than the wii

The non Zelda versions of these games come out on Vita. I'm pretty sure the Xbox 360 solved the how to put hundreds of enemies on the screen at once thing with Dead Rosing back in 2006. Hyrule Warriors tells us nothing about Wii U CPU grunt. However it does have some very pretty games so I can't really say I care what is inside it.

It's about the most CPU-heavy game on the system that wasn't designed around a different architecture. (PS3/360 ports)

Uhmm...similar games have been seen on the ps2, even. Have you heard of Dynasty Warriors? Every single iteration of that game features the same thing, Hyrule Warriors included.

The PS2 iterations had far, far fewer characters in play at once.

It isn't just 3 Wii CPU's together. It's 3 Wii CPU's optimized and overclocked

3 Wii CPUs together would have 768kb of L2 cache, Wii U's CPU has four times that amount. ;)

Isn't that weak compared to?

If your basis is "sheer insane amount of active NPCs it swarms the screen with" then Ikusagami/Demon Chaos completely dwarfs Hyrule Warriors.  Which shows that a simple number of NPCs doesn't tell you much.