Yes, it's in one monolithic file.
Try making a minor typo in the syntax, then restart networking. You will lose all network connectivity on ALL interfaces.
Fix your typo and try to start networking again. It won't work until you reboot.
Or you can try adding your config to one of the seemingly infinite network config subdirectories (ifup.d, post-ifup.d, etc). Make the same typo anywhere in the subdirectories and you'll still mess up all networking and have to reboot.
I guess this is acceptable if you haven't used any OS besides Windows 98. For the rest of us it's maddening.