I think my biggest point of contention is the bolded part. In real life I've spent hours reading and listening to people argue whether or not 9mm vs .45 was superior, or whether 5.56 versus 7.62 was superior. Whether bullet quality was more important, whether bullet placement was the only factor. Whether it was possible at all to quantify stopping power except in the sense that getting shot in the chest by a .50 cal was definitely worse than getting shot in the chest by a .22. In the same way that I would assume that characters would for sure know that greatswords are better than daggers.
To me with all of our anecdotal evidence, ballistic tests, studies, and reports from health physicians. We still can't tell you conclusively what's better when it comes to similar cartridges. I absolutely believe there are characters in DND who will tell you that rapiers are obviously better, because they are bigger. And I will tell you that they will have no hard proof for this statement, and that this statement is not considered true by the world at large.
I don't believe that PCs understand their weapons, better than we understand our own weapons. I also don't believe that PCs would in universe never consider non mechanical factors in their decision making. Because they are not a slab of numbers, they are presumably people who exist in all the spectrum we do not otherwise mention, such as how comfortable they find a particular weapon to use. If every PC knew for a fact the damage values of close(1 damage on average) weapons, then you are meta gaming. Which is okay.
If all fighting characters know as a matter of fact they'd be better off getting hacked at with a scimitar than a rapier. That a trident and a spear might as well be the same weapon. That daggers, clubs, whips, and sickles were all just as deadly as each other. Then you are metagaming. Which is not a bad thing, I metagame on the regular when it comes to certain things that may drive people insane. Of course my character knows that a rapier is better! Mostly to smooth out my gaming experience, in the same way I will share knowledge with another player out of universe before they take an action. You can justify it and say, my character noticed over the years that X weapon was better, or that I as a player was just telling another player something his character should already know. But still that's metagaming, and where you fall on that line is up to you. We may as well argue whether or not OOC conversations are metagaming, or a corrective measure to replace all the offscreen time these characters spend together getting on the same wavelength.