EPERM upon kill(2) of "empty" group

At least on

$ uname -r -v -m
25.2.0 Darwin Kernel Version 25.2.0: Tue Nov 18 21:09:55 PST 2025; root:xnu-12377.61.12~1/RELEASE_ARM64_T8103 arm64

running the Posix program test.c

#include <errno.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <unistd.h>
int main() {
    pid_t ch = fork();
    if (ch == -1) {
        perror("fork");
        return EXIT_FAILURE;
    }
    if (ch == 0) {
        return EXIT_SUCCESS;
    }
    if (setpgid(ch, ch) == -1) {
        perror("setpgid");
        return EXIT_FAILURE;
    }
    siginfo_t stat;
    if (waitid(P_PID, ch, &stat, WEXITED | WNOWAIT) == -1) {
        perror("waitid");
        return EXIT_FAILURE;
    }
    if (kill(-ch, SIGKILL) == -1) {
        perror("kill");
        return EXIT_FAILURE;
    }
    return EXIT_SUCCESS;
}

as

$ clang test.c
$ ./a.out 
kill: Operation not permitted

fails with EPERM even though the lifetime of the process group has not yet ended (due to the WNOWAIT).