PIP was part of CP/M – it was an external executable rather than a command processor builtin, what in MS-DOS terminology is called an "external command", in CP/M terminology a "transient command" – and PIP did have command line options. It didn't use either hyphen or forward slash, instead it put switches (which it called "parameters") in square brackets, for example:
PIP LPT:=X.ASM[NT8U]
Prints the X.ASM file on the LPT: device (line printer). Option "N" means line add line numbers, "T8" means expand tabs at every 8th position, and "U" means convert lowercase to uppercase. [0]
This is different syntax from DEC operating systems, such as RSX-11M [1] or OS/8 [2], where PIP indeed used slash as an option character.
This "switches in square brackets" convention was used by other commands as well, for example the LINK linker [3]. This convention is (obviously to me) influenced by the IBM CMS mainframe operating system, [4] albeit with the parentheses of CMS replaced by square brackets. We know Gary Kildall used CMS at the Naval Postgraduate School, and it also (I'd say) influenced CP/M's use of drive letters A-Z (as in CMS file modes) instead of the multi-character disk device names generally used by DEC operating systems.
Square brackets is CP/M's native convention for command line switches. Forward slash is something not native to CP/M, but a convention spread by certain ISVs. I think the linked blog post is likely right that Microsoft introduced this convention in its products, and I'd say it likely spread to other ISVs due to the widespread adoption of Microsoft's development tools. The fact that many larger ISVs used DEC minicomputers as a cross-development platform, not just Microsoft, was likely also a factor. While IBM mainframes influenced Kildall personally, and through him CP/M, IBM mainframes were much less likely to be used by CP/M ISVs than DEC minicomputers, because they were a lot more expensive.
[0] http://www.tramm.li/i8080/cpm22-m.pdf page 1-33 (PDF page 40)
[1] http://www.bitsavers.org/www.computer.museum.uq.edu.au/RSX/A... page 5-2 (PDF page 70)
[2] https://www.grc.com/pdp-8/docs/OS8_System_Reference_Manual.p... page 65 (PDF page 73)
[3] http://bitsavers.org/pdf/digitalResearch/cpm_plus/CPM-Plus_U... page 83 (PDF page 96)
[4] see for example https://www-40.ibm.com/servers/resourcelink/svc0302a.nsf/pag... page 42 (PDF page 66)