Don't get me wrong, I don't think you need or should need a degree to program, but if your standard of what abstractions you should trust is "all of them, it's perfectly fine to use a bunch of random stuff from anywhere that you haven't the first clue how it works or who made it" then I don't trust you to build stuff for me
The big thing here is that the transformations maintain the clearly and rigorously defined semantics such that even if an engineer can't say precisely what code is being emitted, they can say with total confidence what the output of that code will be.
They may not, but they can be. Buy a book like "Engineering a Compiler", familiarize yourself with the Optimization chapters, study some papers and the compiler source code (most are OSS). Optimization techniques are not spell locked in a cave under a mountain waiting for the chosen one.
We can always verify the compiler that way, but it's costly. Instead, we trust the developers just like we trust that the restaurant's chef are not poisoning our food.
Sure, obviously, we will not undersatnd every single little thing down to the tiniest atoms of our universe. There are philosophical assumptions underlying everything and you can question them (quite validly!) if you so please.
However, there are plenty of intermediate mental models (or explicit contracts, like assembly, elf, etc.) to open up, both in "engineeering" land and "theory" land, if you so choose.
Part of good engineering as well is deciding exactly when the boundary of "don't cares" and "cares" are, and how you allow people to easily navigate the abstraction hierarchy.
That is my impression of what people mean when they don't like "magic".
In that post, the blanks reference a compiler’s autovectorizer. But you know what they could also reference? An aggresively opaque and undocumented, very complex CPU or GPU microarchitecture. (Cf. https://purplesyringa.moe/blog/why-performance-optimization-....)