Nixon did the Saturday Night Massacre against the special counsel that was working the Watergate case. That set of firings (Nixon was referred to as a “Cox sacker”) was effectively used to remove Nixon from office.
Old pepole and those who study histlry got the association. Imagine if Trump had fired Mueller and Rosenstein. He certainly would have been in the right doing so.
So, POTUS’ reference was that he doesn’t fire people when they are in the middle of investigating him. Thank you!