The master/slave terminology has a history in technological circles, typically being used to describe situations where one device is used to control another device. It actually predates computers; you can find examples in mechanical and hydraulic machinery going much further back.
As far as that context goes, the term is accurate for what RATs do: the hacker's "master" device can be used to control what the "slave" machines do. I assume that this is the context in which these people are using those terms. But it is, admittedly, somewhat jarring for people unfamiliar with that context.
While the technical meaning may be part of why they are using it, I think they are clearly using it mostly for its connotation related to controlling a person.
It's true that it has history in technological circles, but the politically correct wording appears to now be "primary" and "secondary". Interestingly, in 2003:
"Los Angeles officials have asked that manufacturers, suppliers and contractors stop using the terms "master" and "slave" on computer equipment, saying such terms are unacceptable and offensive." [1].
As far as that context goes, the term is accurate for what RATs do: the hacker's "master" device can be used to control what the "slave" machines do. I assume that this is the context in which these people are using those terms. But it is, admittedly, somewhat jarring for people unfamiliar with that context.