Yeah, I came into it from a Castlevania mindset: the land is overrun with horrors and you are the only ones who can stop them. It's Fantasy with a coat of black paint and some gothic tropes rather than a emulation of a real horror genre (something D&D is a poor match for due to it's emphasis on heroic fantasy).
Don't get me wrong, Castlevania was a great game (that was up with with games like Kings Quest and Bards Tale for me growing up). But I definitely did come at this as a kid who watched a lot of old horror, and my entry point was actually Knight of the Black rose. For me that wa sa bit instructive because, I knew Soth, not Strahd, and Soth was the ultimate bad-ass in Krynn, a sort of Darth Vader figure. To see his frustration as powerful as he was, not be able to deal with a simple vampire, was fascinating. And I think this gets at how black box approached the issue of emulating horror in D&D. Let me preface by saying I think there is truth in what you say, when I made my horror RPG, I set PC health very low, and while characters could advance up to ten levels (totally different system than D&D keep in mind), I kept a close eye on how powerful they got because I felt it was important for horror, to have PCs that are vulnerable to it. So I definitely this "D&D can't do horror" has a strong grain of truth to it. What is brilliant about black box and some of the early supplements that really embraced the black box philosophy is how well they undermine D&D's ability to nerf horror. And I think a big part of that, was the Soth effect, taking characters who should otherwise be powerful, but have the physics of Ravenloft itself working against them. You see this not only in how spells were adjusted, but also in how monsters were very customizable and how their immunities were often extensive. It was very easy to make a monster that powerful characters had a hard time affecting or killing permanently unless they 1) found the right tool for the job, or 2) found a way to undo whatever curse was plaguing the creature. Again, definitely not quite the same as being super vulnerable to powerful foes, these still are characters with impressive ablates and lots of HP. But even advancement was slow in Ravenloft because you just weren't having the same number of encounters (it was more about quality of individual encounters than quantity), and things like magic items were so much more scarce. It definitely took some work. Wasn't perfect, but I do remember being very impressed with how it managed to make D&D work with horror.
EDIT: Also to be clear, I am not knocking your opinion. I think it is interesting we have such strikingly different takes and views on Ravenloft. But also I think I understand where you are coming from (you are presenting a very consistent perspective on the setting).