By definition, it's not. But it shows that there are levels of metagaming. The first level is "metagaming as required to keep the game flowing". This includes players knowing what their actual modifiers are, knowing monster AC's, and not turning a troll encounter into a TPK because no one knows how to kill a troll. This kind of metagaming is rarely bad.
The second level I'd call "incidental metagaming". This is when a player acts like their character lives in the game world, and will spout out facts that are known by many, even if it was never established that the character knows these things. Saying you know about the War Wizards of Cormyr, Bladesingers, or Hobgoblins, for example. This kind of metagaming is not often bad.
The third level is "restricted metagaming": here, the player is taking knowledge that is only known to a privileged few and using it or acting upon it. Knowing that the phylactery of a lich is his massive golden throne, or that the Princess is secretly a Silver Dragon. This kind of metagaming is often bad.
And finally, we have "terminal metagaming". This is where you've read the adventure or the DM's notes, or took something the DM told you in confidence and bring it into game. Once, my roommate used my computer when I was at work, and saw a lot of word docs on my desktop. Curious, he read them, and they were stories about my campaign's lore.
When he mentioned them to the other players, I was not happy. Terminal metagaming is almost always bad, but it doesn't have to be- it's annoying, and a breach of trust, to be sure, and there should be a discussion about it. Where it crosses the line is when the player uses this information to their benefit, like knowing to search a certain area to find a magical intelligent sunblade (I'm sure some of you know what adventure I'm referring to).