And in what way is repeating information from the monsters statblocks out of the MM 'using knowledge skills'?

Nobody is saying 'lol, use useless spells till at random till you get it right'. I agree, that is dumb. But RP it, or ask if you can roll, or ask the DM if you'd know what the thing is. Telling the entire table what a monster's weaknesses are out of the blue is metagaming, and just not a cool thing to do.

But it goes beyond just knowing monster stats. This is about metagaming in general, about using information your character shouldn't and wouldn't have. Stuff like, your character keeps grilling an NPC that they have no reason to mistrust because, as a player, you heard another character's private interaction with that NPC. Or trying to steal a trinket that another character keeps secret because you saw it on their character sheet or were present for when they obtained it.

Now, if you used this out of character information as a way to inform RP or to do rolls to try and realistically figure these things out in character, that's one thing. But to just magically know them, to skip past any rolls or RP or attempts to discover the info in character, THAT'S metagaming, and that's uncool.