I'm not constructing a strawman. Please try to understand that we're talking about two (maybe three) different meanings of the word "metagame" here, and only one is pejorative.
Metagame also refers to "the game outside the game". If I ever have a thought like "DM Melissa and I are experienced old hands, but everyone else at the table tonight is a newbie, so this session will probably be pretty easy and light." that's a metagame thought, and any decisions I make in the game based on that information are functionally metagaming, but that's not the same kind of metagaming as reading the module ahead of time and knowing that these trolls are actually immune to fire, but acid affects them normally.
As others have pointed out earlier in the thread, if I think (or say) "Wow, Joe seemed to know exactly where to search for that secret door! He must be cheating and metagaming by having read the module*" that is itself metagaming. That's me NOT inhabiting my character, but instead engaging in out of character speculation about how someone else is playing the game, at the cost of my own fun!
(*ironic in the initial example, of course, because in the real example the player actually suspected a trap, guessed where one might be, Investigated, and turned up a secret door he wasn't expecting.)