Note that the OEM README[1] for MS-DOS 2 (the first version with a hierarchical filesystem) suggests that it was at IBM’s insistence that MS-DOS used backslash for directories and slash for options, rather than slash for directories and hyphen-minus for options (even though Microsoft’s own programs for CP/M apparently used slash for options due to DEC influences, instead of the brackets used by CP/M itself[2], so this is a bit of a tangle). The documentation and examples for CONFIG.SYS[3] also bear an evident Unix influence, note in particular AVAILDEV and SWITCHAR. The MS-DOS-specific calls for file descriptors (as opposed to CP/M-compatible calls for FCBs) are called “XENIX file calls” in the source[4]. (All DOS system calls for files accepted either separator, but the option parsers would retrieve SWITCHAR[5] and give it precedence.)
[1] https://github.com/microsoft/MS-DOS/blob/master/v2.0/source/...
[2] https://news.ycombinator.com/item?id=30510935
[3] https://github.com/microsoft/MS-DOS/blob/master/v2.0/source/...
[4] https://github.com/microsoft/MS-DOS/blob/master/v2.0/source/...
[5] http://www.ctyme.com/intr/rb-2752.htm