DOS system calls accept either path separator (and so does Win32). But since a leading '/' (and even an embedded '/' in some cases) is treated as introducing an option, it cannot really be used as a path separator.
MS-DOS 2.0 and some later releases supported changing the option character, typically to '-', resulting in a more Unix-like behaviour. However, command line parsing was left to each program and there was no widely used getopt() function, so this was never universally supported. There was also no convention of a special argument like '--' that would disable option parsing in subsequent arguments