It is true that the CP/M operating system itself did not use the forward slash, but the only reason for that was that the operating system included only a handful of executable programs, the main being the equivalents of the later "COMMAND.COM", "DEBUG.COM" and "COPY.COM" of Microsoft.
The very few executable programs provided with the OS did not have any command-line options. A few of the most common file operations were shell built-ins, not separate executable programs.
On the other hand, most CP/M executable programs (which were bought from third parties, not from Digital Research, the vendor of CP/M) had command-line options and, without exceptions, they used the forward slash for command-line options, instead of the hyphen used by UNIX.
It would have been absolutely impossible to use CP/M without being aware of the convention that the forward slash is used for the command-line options.
The article is completely correct that the use of the forward slash for options was not introduced by CP/M, but it was taken from the operating systems of the DEC Corp., which, during the seventies, were much more widely used than UNIX, which was an alternative for them only on PDP-11, and it was not yet used much outside the Bell Labs.
Nevertheless, all the programs ported to MS-DOS 1.0 (e.g. MS BASIC, Turbo Pascal, WordStar, dBASE and so on) had not been ported from programs written for DEC OSes, but from programs written for CP/M.
There is no doubt that the forward slash of MS-DOS 1.0 is directly inherited from CP/M. It was used by the same programs, for exactly the same options, so they must have kept the command-line parser without changes. I have used many of the same programs both first for CP/M and then for MS-DOS, so I can be certain about this.
I have also used the DEC RSX-11M operating system on PDP-11, and indeed the RSX-11M utilities, for example PIP (Peripheral Interchange Program, a file transfer program with many options, somewhat like UNIX dd), used the forward slash many years before CP/M and most programmers for CP/M should have been familiar with this convention, so they have reused it, but the relationship from DEC OSes to MS-DOS 1.0 is indirect, via CP/M.