Well, I do think people forget that defense is more than just straight up guarding your man. Creating turnovers is one way to improve defense, and he doesn't do it. Castle is great at defending his man, and that's important, but I also think it's probably the part of defense that gets too much credit. There is so much more to defense than that. Do I think Castle is bad at those other things? Not really, but stocks is a great proxy for that stuff.
If the indicator wasn't so extreme, like if he was bottom 30% instead of bottom 5%, it wouldn't bother me as much. Talking through his specific scenario makes it reasonable, but wouldn't you think there would be so many other cases in history? Like I said, it's not enough for me say he won't be a good defender. I have never said that. But we're talking about the elite of the elite and getting offended that I'm not saying he's like top 1 guy in the NBA....I don't think he's there.
To answer your question, it's not really a problem if you're talking about him being a good-great defender. If you're talking about him as the top 1 defender in the league and the best you've ever seen, yeah it is a problem in my eyes.