That's just some injected opinion, I assume from someone contributing to glibc who doesn't like posix_spawn I guess? In any case it is wrong.
Don't assume what is written in man pages is the truth. Some of them have a lot of opinion added. It can be useful to cross-check man pages between systems - they don't always call out non-portable options or behavior.
On some kernels posix_spawn is a syscall or specifies flags that make it more efficient than fork+exec. Darwin is one such system, though you can use POSIX_SPAWN_SETEXEC if you still want to replace the current process with a new executable rather than creating a child.
Don't assume what is written in man pages is the truth. Some of them have a lot of opinion added. It can be useful to cross-check man pages between systems - they don't always call out non-portable options or behavior.
On some kernels posix_spawn is a syscall or specifies flags that make it more efficient than fork+exec. Darwin is one such system, though you can use POSIX_SPAWN_SETEXEC if you still want to replace the current process with a new executable rather than creating a child.