At a high level, isn't this like implementing your own "malloc" and "free" that just pulls from your process's own memory pool instead of the OS? Or is there more to it than that?

No, it's just placing the appropriate structs and buffers on the stack (when not provided by the caller).

It does eliminate a certain couple classes of errors, and makes some others less likely.

I didn't read all the code, but I don't think it's using alloca or the like. So the stack allocation sizes are known at compile time, and bounded unless there's some recursion going on (which is unlikely).