I'm not too terribly surprised by this, to be honest. Interesting, still, but not surprising. What's not said is that he's not doing strength and conditioning, NOW.
GSP is a top tier fighter who's been studying martial arts for 20+ years, and has been fighting professional for, I think, eight years or so. And during that time, he was probably doing strength and conditioning. Hell, you can see training montages on youtube that include him strength training. However, the dude is probably strong as an ox now, and is in a lower weight class. What that means is that he literally can't put on much, if any, more muscle without being forced up in weight.
Cool find though.