It's really both. They have good, high-poly models and high-resolution, well-designed textures, and they also know their way around shaders and tweak cool realistic effects until it looks good. From experience, you can't just put one model there, shade it, and expect it to look amazing. There's theory... and there's implementation, where you mess around with the graphical parameters until you get the right effect for your game.
Just like you can take a random photo outside, and it will look decently good, but you will probably want to post-process it in Photoshop, trying various options and filters until you get something that actually looks impressive.