Meh, I think GSP is only saying that because he compares Lee to himself - an explosive wrestler with fast strikes. He's overlooking two huge differences; fight IQ and pacing. Lee has proven time and time again that he'll just run in for a double with little thought of a setup and muscle his opponent to the ground. He seems to approach every fight the same, and when he gets a top wrestler, he hits a massive wall and gasses like crazy.
His training probably has something to do with it, but I don't see what GSP is talking about. He's composed, only uses explosive shots when he's timed them perfectly (his take downs take 20% of the energy of Lee's because of his setups). Lee strikes me as a guy that got too much hype too quickly and won't adapt because he spent too long thinking he knows better. If you're going to fight like RDS, you better build the same conditioning as him. Their fight was a perfect example of how Lee just isn't at that level with his conditioning.