The config and System.map files are located in /boot because they are installed there by Linux’s make install … and some tools then expect to find the files there (depmod for example expect to find System.map in /boot).
It’s unlikely the boot loader (grub) has a need to access kernel configuration (/boot/config) or symbol tables (/boot/System.map) in order to boot a Linux kernel.
Perhaps userspace need them later during initialisation. I will do the test.