From reading the synopsis of the plot (I've seen Pi several times, but never heard of Tetsuo: the Iron Man before), what does Tetsuo: the Iron Man have to do with Pi? The story at least shows no obvious connection.
It's a question of degree, and also one of context. In this case, he seems to have extracted a lot of value from somebody else's work without himself having created much that is new.
But I'd note that whenever someone points out that X copied Y, we can always go deeper and find out that they both really copied Z, who copied A, ...
There's no such thing as original art.
Everything is a copy.