I have always understood that C generally compiles almost directly to assembly with little to no abstraction overhead, and it would not require platform-specific ASM code.

You have always understood incorrectly then. I'd recommend a trip over to Godbolt and take a look at the assembler output from C code. Play around with compiler options and see the (often MASSIVE!) changes. That alone should tell you that it doesn't compile "almost directly to assembly".

But then note something different. Count the different instructions used by the C compiler. Then look at the number of instructions available in an average CISC processor. Huge swaths of the instruction set, especially the more esoteric, but performance-oriented instructions for very specific use cases, are typically not touched by the compiler.

In the very, very, very ancient days of C the C compiler compiled almost directly to assembly. Specifically PDP-11 assembly. And any processor that was similar to the PDP-11 had similar mappings available. This hasn't been the case, however, likely longer than you've been alive.