In the spirit of being charitable, I would say that the article is highlighting a place where the guarantees are incompletely documented. And not having a solid specification and clear documentation is absolutely one of the things that makes unsafe Rust hard.

I'd actually like to qualify that a bit. Doing unsafe is not especially hard, just use pointers everywhere instead of references. That's exactly like C (which doesn't even have references), but the syntax is clunkier, you have to use function calls instead of concise operators like * and ->. What is hard is finely interleaving safe and unsafe Rust, as the author is trying to do. That's difficult because the unsafe code has to upload all the safety invariants of safe Rust, and those are indeed complicated.