Most of these are due to last minute changes to achieve a greater degree of compatibility with IBM's implementation of MS-DOS (PC DOS). This includes the use of "\" instead of "/" as the path separator, and "/" instead of "-" as the switch character.
https://github.com/microsoft/MS-DOS/blob/80ab2fddfdf30f09f0a...
all of it came from QDOS, a CP/M-like OS M$ bought based on hiding their plans, and that IBM made PC-DOS from. PC-DOS shipped with the original IBM PC.
MS-DOS only became a thing once there were "IBM PC compatible" clones. It was very smart of M$ to see that coming. They kind of rode IBM's coattails into dominance in the market. Back then, personal computers were oddities and when IBM entered the game, their reputation gave them credibility.
I don't think 86-DOS (aka QDOS) itself had slash-based options. At least, in a quick skim of the 86-DOS 0.3 user manual I couldn't see any. [0]
But they were there in PC-DOS 1.0, which suggests Microsoft added them after licensing 86-DOS.
[0] https://web.archive.org/web/20160409042609/http://www.paters...