C is syntactic sugar on top of assembly

That’s part of what C does, but it’s not the only thing.

Assembly adds mnemonics and a logical structure to machine code.

C largely does the same, but adds a cross-platform abstraction, mechanisms for organizing and sharing source code, a standard library, and various other things.

Syntactic sugar is such a broad and vague term that those features you mentioned could also technically be considered syntactic sugar. Instead of having to write harder longer code you can write easier and less code.

I think we can understand syntactic sugar as an alternative (hopefully more convenient, elegant, or pleasing) syntax for expressing something that can already be expressed.

That is, something that works the same but looks better (hopefully).

A cross-CPU abstraction, a preprocessor, a standard library all have certain elements of syntax in them, but go well beyond alternative syntax.

Assembly is syntactic sugar on top of Machine Code