I really don't want to wade into this mess, but the last bit here really got me. This is one of those, "too clever by half" arguments that seems great until you unpack it.
Think of it in this context-
Player A cleverly defeats an unpublished monster immediately using it's special code word, "November," that only is in the DM's written notes, and that Player A could only have known by reading them when everyone went to pick up the pizza & beer.
But Elfcrusher responds, "You can't worry about that, because then YOU ARE THE ONES METAGAMING, since you all know that in real life, Player A was alone with the notes while you went for pizza and beer!"
/facepalm
As I wrote above, there are degrees of metagaming (from outright "cheating," aka reading the module ahead of time) to bad behavior (reading up on the monsters you know you will encounter) to ¯\_(ツ)_/¯ (if you've ever played D&D, you probably know something about D&D trolls and fire) to inevitable knowledge (D&D is a game, with game mechanics), and different tables handle it in different ways.
That said, this isn't a good argument. No offense, but it's kind of third grade, "I know I am, but what are you!!!!"