As others have put into footnotes, "noob" means someone who is not new to the game but actually just so unskilled that they SEEM new to it, thus the different spelling from "newbie" which refers to someone actually new and having a reason (apart from the "noob's" implied stupidity) to be bad at the game by comparison. It's very important to understand this difference in the parlance. It's all about the comparison between the one saying it and those they're playing against, and how subjectively bad those opponents seem to be. It's not about their objective skill levels.
"I pwn pros" isn't a perfect line either. To "pwn" is to completely destroy an opponent, usually in a humiliating manner (a common colloquial replacement for "pwn" is to "cream" where I'm from). If you destroy an opponent in the way "pwn" implies, they probably don't count as another pro. You could say it if you were a pro among pros (like the world or at least national champion at any game), but that would make it a very over the top phrase for anybody else to use.