I don't disagree with this statement. My question is, how is this good role-playing? Because a professional fighter, who knows they might die if their skills aren't sharp enough, has plenty of incentive to be the best fighter they can be. Disregarding that incentive seems like a pretty egregious case of meta-gaming.
Or I guess you could just play it that they're super lucky, and they happen to not run into situations they can't handle, but then you're basically deciding that they should win by your decision of what to put them against. And in any case, it's still meta-gaming, because you have to play the world and the NPCs inauthentically in order to contrive those situations.
Meta-gaming is bad. That truth is unquestionable. It follows directly from the definition of role-playing - that decisions are made from the perspective of the character - and it's not something that you can change while you still claim to play a role-playing game.
If you want to meta-game at your own table, then fine, whatever. When someone comes into a public forum and claims that meta-gaming is a valid way to role-play, then that's an issue, because it damages the integrity of the role-playing community. Role-players need to know that the GM isn't going to cheat, which meta-gaming very much is, when you're playing a role-playing game.
I disagree that meta-gaming is equivalent to cheating. I also disagree that tailoring the campaign to the PCs qualifies as meta-gaming. I disagree yet again that metagaming by the DM is inherently problematic when it is used to maximize the players' fun. I elaborate on each point of disagreement below.
Regarding meta-gaming being equivalent to cheating... consider a player who evaluates (from the perspective of their character) that the best chance of success comes from splitting the party. The player opts not to push for this course of action because splitting the party would make the game less fun for that particular group. Is it meta-gaming? Yes, by definition. Is it cheating? Absolutely not. Indeed, many consider it poor form to make IC decisions that detract from other players' fun. (Consider, e.g., stealing from the party with the justification that "it's what my character would do".) Ergo, meta-gaming is not equivalent to cheating.
Second, out-of-character decisions, by definition, can't qualify as meta-gaming. Tailoring the campaign to the party usually involves the DM making out-of-character decisions about the content of the game world. Yes, those OOC decisions should be made with an eye towards consistency with the IC events of the campaign thus far, but that still leaves a huge range of possibilities for the DM to select from. Selecting those options suited for a particular party cannot be meta-gaming, by definition, because it wasn't an in-character choice to begin with.
My answer to your question regarding the fighter and roleplaying is similar. To the extent that character-building choices reflect IC decisions, the character doesn't know what is mechanically optimal because the character is unaware of the game mechanics. The choice whether or not to optimize is therefore largely an OOC choice. Sure, there is an IC component too, regarding how the character wants to spend their time. But I don't accept that the only IC choice that qualifies as "good roleplaying" is "to be the best fighter they can be". Characters, like people, are multi-faceted, and it's not bad roleplaying to play them that way. Would you really argue that a fighter who spends one of his ASIs to take the Gourmand feat, for example, is roleplaying poorly because IC they spent time to learn how to cook rather than spending that time to be the best fighter they can be?
Third, I stridently disagree that DMs always need to avoid meta-gaming. At some tables, such as yours, they absolutey do need to avoid metagaming,
because DM metagaming would clearly detract from the fun of the players at your table, and detracting from the fun of the players is a bad thing. But at tables like mine where metagaming is an expected (and explicitly acknowledged) tool in the DMs toolkit, DM metagaming can instead enhance the fun of the players, and therefore can be a good thing. (Like all such DM tools, care should be taken not to rely on metagaming too heavily, lest it lose effectiveness.)
Finally, I'd like to point out that your post came across as a personal attack on my integrity, and as an accusation that my prior post "damag[ed] the integrity of the role-playing community". We merely have different playstyles, and while I am happy to discuss the differences with you, and recognize that you don't
like my playstyle, I would ask that you refrain from insinuating that my playstyle is inferior (let alone that my choice to discuss my style publicly is damaging to the community itself).