I made no comparison between “defer” and the current C situation.

In comparison to RAII, “defer” as a language feature is a kludge. Mainly because RAII completely solves the problem of correct resource cleanup for API users while “defer” does not.

RAII is a delight when it comes to local heap memory. However, if you want to pair fclose with a fopen call then you need to wrap fopen in a type and make the destructor call fclose. This is totally fine and I like it a lot in C++ and Rust, but it doesn't gel with existing C apis. Therefore I think defer is a good choice for C regardless of what I might choose were I to be implementing a language from scratch.

RAII is an idiom, not a language feature. Destructors might be the language feature, but they have their own issues and caveats.

So I don't see the comparison...

I think it's quite reasonable to compare defer and destructors, in that case.

Destructors probably cannot be usefully imported into C while preserving the simplicity of the language: then you'll want at least unique_ptr to manage your memory with destructors, then probably shared_ptr, and some ownership semantics as you pass those around, et cetera. In that sense, defer strikes a balance between simplicity and usefulness. However, there is still a comparison.