Right, but having a dummy plug will cause the vast majority of laptops to disable their built-in microphones and instead listen to whatever you plugged in. (In the case of a dummy, this would result in a dead line.)
The idea is to fool the system into having no physical microphone, and even a hacker/malware that compromises the operating system is going to have a hard time dealing with whatever soundcard firmware is responsible for making that switch in the hardware.
On many older devices the signal from the internal microphone is routed through the jack, just like the speakers often connected through the headphone jack. Plugging a device in physically breaks the connection to the mic/speakers in those cases.
That does not apply to most modern computers though, and especially not anything with a four-pin combo jack (including that Macbook). Those have to be able to handle situations where normal headphones are plugged in and the internal mic is still being used, so the mic switching is entirely software on these.
Headphone/speaker switching may be in hardware, but a lot of laptops I've used under Linux allowed me to treat the headphone jack as an entirely separate audio device. I suspect the Macbook would allow the same, though I don't have a new enough one available to test. Mine's an '08 model that Sierra finally EoLed.
The idea is to fool the system into having no physical microphone, and even a hacker/malware that compromises the operating system is going to have a hard time dealing with whatever soundcard firmware is responsible for making that switch in the hardware.