Omak Darkleaf
Oath of Sloth
It's true that "show don't tell" has been a fiction writing maxim "for decades," but—ironically—you're arguing for its opposite. Indeed, the show does show that cortosis exists: we see The Stranger use it to parry and disrupt the lightsabers of the jedi, and their surprise is clear by the acting. Your actual complaint appears to be that we're not told what cortosis is: you want it to be named and described by the characters. In my estimation, it would be poor storytelling to do so.
You're entitled to your preference, of course, but I don't think that it's an error of craft that the show is showing rather than telling. It's more immersive to witness the shock of the jedi and to see their reaction. Their not knowing that it is cortosis and our not knowing that it is cortosis heightens the tension and creates a mystery and an obstacle. To better fight The Stranger, the jedi now need to investigate the phenomena to learn about it (for their sake and ours) and how to counter it.
Good. With two degrees in creative writing, surely you would concede that many effective narratives withhold information from the reader that the protagonists themselves are not privileged to. Even if it's not your aesthetic preference, this is something that most narratives do, and not all of them are considered to be poorly executed.
Consider Hemingway. The Sun Also Rises and "Hills Like White Elephants" are two of his best examples of his "iceberg theory" in which the only a small fraction of the story is visible above the waterline and the bulk of it lies beneath the waves, where it needs to be inferred. Some people don't enjoy Hemingway's aesthetic, but that's a subjective appraisal. The contemporary critical consensus is that Hemingway was, when he was writing his best prose, a master craftsman.
We have no reason to believe that these jedi know what cortosis is and every reason to believe that they do not from their reaction to its effect on their sabers. But, if they did know, they would have no reason to explain what cortosis is to each other—only to us, in which case the show would be using exposition (a form of telling) rather than showing us an explanation such as Sol investigating the mystery and learning its answers in the jedi archives. It's good he doesn't know everything, isn't it? We don't want our jedi to look like know-it-all Mary Sues, do we? Moreover, breaking into the fight scene with a line of exposition would slow the pace of the narrative. There have been plenty of instances in which your fellow critics have complained about what they already perceive to be a slow pace. Let's not bog down the narrative for their sake.
I personally don't mind "intertextuality," which is as old as extant narrative itself. Homer doesn't both to explain everything needed to fully contextualize his stories. The audience is expected to bring knowledge from other epics to The Iliad to fully understand it. This approach to narrative has been prevalent in the Star Wars universe since at least the prequels. To expect Star Wars to suddenly abandon Easter Eggs is unrealistic—but at least I think we can all agree that there are fewer of them in this show than in most.
And, all that said, you're making one assumption that some of don't. You take it as a given that we're not going to learn what the deal with cortosis is. I would be highly surprised (and bothered, yes) if we don't eventually learn about it. These episodes aren't episodic; this is a serialized narrative, and we need to watch them all for the complete story and a complete context to view the entire story in.
Reading through this thread, I'm left with one major supposition. There critics of this show are not watching it in good faith, whereas I am. Thus far, I trust the director and the writers to take me to a satisfactory conclusion. I've yet to see what I consider to be major errors of craft in the narrative and I'm willing to give this show the benefit of the doubt. It's obvious that some other viewers feel the opposite way, and, if it's their thing to hate watch Star Wars shows, that's cool with me. It's not mine, though, and that should be cool too.
