It is, just the existence of goto makes control flow significantly harder to understand. People complain about exceptions in C++ obfuscating control flow, but then they recreate exceptions using goto. The funny thing is that exceptions are just fancy goto, the assembly is almost the same.
The bigger picture of C as a language is not that it's simple, because it's not simple at all. It's inept. It doesn't give developers the tools to write simple code. So easy things become hard, and we sort of jank together solutions that kind of work but usually don't.
I like to compare it to building a shed with power tools versus only a screwdriver. Is a screwdriver simpler than a power saw and all that? Of course. Now think about building a shed. Is it simpler to do with a screwdriver? No. It's much, much more complex. You have to develop complex processes to make that work, and it's not intuitive at all.
C is a language that already makes use of implicit control flow A LOT. I don't see defer being a problem. The irony is that if C just supported these use cases out-of-the-box, it would be simpler and easier. As a concrete example, consider polymorphism in C versus C++. Both languages can do it, but one provides the tools and one doesn't. In C++ I can go to definition, I can concretely define what polymorphism is allowed and what isn't, and the type system gives me the tools to make it safe. In C, none of that is true, so when we do polymorphism with function pointers, it's much harder to understand what's actually going on, or what could be going on.